Lesson Explainer: Fertilization and Pregnancy Biology

In this explainer, we will learn how to describe the process of fertilization and subsequent embryo development, including an outline of how twins may form.

Humans reproduce sexually, which means that a diploid zygote is formed through the fusion of haploid male and female gametes in a process called fertilization. The processes of fertilization and the 9-month period of the mother’s pregnancy are controlled by several systems working in harmony to create the perfect conditions for a developing fetus. In this explainer, we will understand the fascinating processes of fertilization and pregnancy at the cellular level and learn about the various organs involved.

Key Term: Fertilization

Fertilization is the process of combining a male sperm cell with a female egg cell to produce a zygote.

Key Term: Pregnancy

Pregnancy is a term used to describe the period during which a fetus develops within the mother’s uterus.

Before we examine the events that take place during fertilization, we need to have a clear picture of the structures of the male and female gametes, or sex cells.

Let’s begin by taking a look at sperm cells. Sperm are the male gametes that are composed of a head, a neck, a midpiece, and a tail, surrounded by a plasma membrane. The head of the sperm cell, as depicted in Figure 1, contains a haploid nucleus (23 chromosomes, including one sex chromosome) and a cap-like structure called the acrosome. The prefix acro- means “end” or “extremity.” The acrosome contains enzymes that help the sperm cell penetrate the egg cell, as we will learn later on in this explainer. The neck contains a centrosome made of two centrioles, which are essential after the process of fertilization for the early division of the zygote. The midpiece is packed with several mitochondria, which produce the energy required by the tail, or flagellum, to propel the sperm cell forward in a corkscrew motion. The total length of a single human sperm cell is close to 50 micrometres.

Key Term: Acrosome

The acrosome is a cap-like organelle that coats the anterior portion of the head of the sperm cell and contains enzymes capable of digesting the coating of an egg cell.

Figure 1: A diagram depicting the structure of a sperm cell (the male gamete).

The female gametes are called ova or egg cells. These cells are considered among the largest cells in the human body: about 0.1 mm in diameter, which is much than a sperm cell. As you can see in Figure 2, the plasma membrane of the egg cell is surrounded by two layers. The first layer is a clear, jelly-like coat called the zona pellucida, which is made of molecules called glycoproteins. The word pellucid means “translucently clear.” The second layer, which surrounds the zona pellucida, is an outer layer of cells called the corona radiata. These layers have protective roles for the ovum and regulate its interaction with sperm cells during fertilization, as we will learn later on. The ovum has 23 chromosomes, including one sex chromosome, which is an X chromosome. It is important to note that an ovum will never carry a Y chromosome.

Figure 2: A diagram depicting the structure of an egg cell (the female gamete).

Key Term: Egg Cell (Ovum)

An egg cell is the female reproductive cell, or gamete.

Key Term: Zona Pellucida

The zona pellucida is the clear glycoprotein layer that surrounds an egg cell.

Key Term: Corona Radiata

The corona radiata is the outer layer of cells surrounding the zona pellucida of an egg cell.

Let’s quickly understand the structure of the female reproductive system. As you can see in Figure 3, the main organs of the female reproductive system are the ovaries, the fallopian tubes, and the uterus. Let’s go over each of these organs and understand their functions.

Figure 3: A diagram showing the structure of the internal organs of the female reproductive system, including the ovaries, the fallopian tubes, and the uterus.

In females, the ovaries play a crucial function in reproduction, as they release the egg cells. They also behave as endocrine glands, secreting the female hormones estrogen and progesterone. The ovaries are connected to the uterus through the fallopian tubes, which are also called the oviducts, as you can see in Figure 3. The fallopian tubes are each around 1012 cm long in humans and are lined with cilia, which move the egg cell toward the uterus.

Key Term: Ovaries

The ovaries (singular: ovary) are the female reproductive organ from which egg cells and hormones are released.

Key Term: Fallopian Tubes

The fallopian tubes link each ovary to the female’s uterus. After ovulation, the egg cell travels along the fallopian tube toward the uterus. If an egg cell is fertilized by a sperm cell, this will likely occur in the fallopian tube.

The uterus, or womb, is a hollow organ where, if fertilization were to occur, the embryo would be implanted in the inner lining, which is called the endometrium, and supported throughout its growth and development into a fetus. The walls of the uterus are made of muscles, which contract strongly during labor. The uterus is actually one of the strongest muscles in the female body. The uterus opens into the vagina through a narrow canal called the cervix. The wall of the vagina has folds that allow it to be stretched and expand during birth. This lining is kept moist and lubricated by glands located in the cervix and on either side of the vaginal opening.

Key Term: Uterus

The uterus is a hollow organ in a female’s pelvis in which an embryo grows and develops.

Key Term: Vagina

The vagina is a muscular tube in the female reproductive system that leads from the external genitalia to the cervix.

You may recall learning about the menstrual cycle. Generally, the female reproductive system undergoes changes through periodic cycles that are regulated by hormonal changes. Notably, estrogen and progesterone are hormones released by the ovaries that modify the endometrium during each cycle. Its thickness and blood supply are increased to prepare for an embryo to implant if the egg cell is fertilized by a sperm cell. While the menstrual cycle is typically about 28 days long, longer or shorter cycles are also normal. In general, the menstrual cycle may be anywhere between 21 and 40 days long.

In the middle of the menstrual cycle, around the 14th day, one of the ovaries releases a single egg cell, in a process called ovulation. Usually, the two ovaries release a single egg cell alternately over successive menstrual cycles. When an ovary releases an egg cell, it travels along the fallopian tube to the ampulla, as labeled in Figure 3. The egg cell releases chemicals that attract the sperm cells toward the ampulla, which is usually the site of fertilization, as we will learn later on.

Key Term: Ovulation

Ovulation is the part of the menstrual cycle when an egg cell is released from one of the ovaries.

During copulation, which is also called sexual intercourse (or coitus), the male inserts his penis into the female’s vagina. Upon ejaculation, hundreds of millions of sperm are released, which travel through the vagina and the cervix into the uterus. Only about 200 of them manage to make their way to the fallopian tube.

Key Term: Copulation

Copulation, or sexual intercourse, is the biological process in which the male inserts his penis into the female’s vagina.

Millions of sperm are lost along the journey from the vagina to the fallopian tube. Sperm may die due to their short lifespan (around 2 to 5 days inside the female reproductive system) or due to the conditions within the female reproductive system, such as the temperature or the vaginal immune defenses. Some sperm may also have low motility, which means they cannot swim all the way to the fallopian tubes and make it to the egg cell. Thus, a male is likely to be considered infertile if his semen count is lower than about 10–20 million motile sperm cells per millilitre of ejaculate.

Ultimately, if a sperm cell manages to make it to the fallopian tube, it travels to the ampulla. Remember that ovulation, or the release of an egg cell, happens once in every menstrual cycle. If ovulation occurs and there is an egg cell in the ampulla, the sperm cell is attracted toward it, and fertilization may occur.

Example 1: Identifying Where Fertilization Takes Place

The figure shows the front view of the female reproductive system. Where does fertilization usually take place?

Answer

Humans reproduce sexually, which means that a human zygote is formed when a male gamete (or sperm cell) and a female gamete (or egg cell) fuse with one another. This process is called fertilization. Copulation is the process in which the male’s sexual organ, or the penis, is inserted into the female’s vagina, which is labeled as D in the diagram above. If ejaculation occurs during copulation, hundreds of millions of sperm are released into the female reproductive system. The sperm then begin their journey toward the egg cell, aiming to achieve fertilization.

Now that we now where the sperm come from for the process of fertilization, let’s think about where the female gametes, or the egg cells, are produced and released. The ovaries, which are labeled as E in the diagram above, release a single egg cell every menstrual cycle. This egg cell is collected by the fallopian tube, which is labeled as C in the diagram.

After the sperm are released into the female reproductive system, they travel through the cervix (labeled as A) and the uterus (labeled as B) to meet the egg cell in the fallopian tube. Here, if a sperm cell manages to reach the egg cell, fertilization may occur.

The part of the female reproductive system in which fertilization occurs is, therefore, the fallopian tube.

Let’s now understand the events that happen at the cellular level when a sperm cell meets the egg cell.

Earlier on in this explainer, we talked about the acrosome, which is a cap-like structure in the head of the sperm cell that contains degradative enzymes. These enzymes are responsible for breaking down the layers surrounding the egg cell.

Around the corona radiata, there is an extracellular matrix that is composed mainly of a compound called hyaluronic acid. To disintegrate this layer, the acrosome releases the enzyme hyaluronidase, which breaks down hyaluronic acid.

After the corona radiata, the sperm cell encounters the zona pellucida, which is a clear, jelly-like layer surrounding the egg cell, as shown in Figure 4. Here, the molecules on the surface of the head of the sperm bind to the molecules on the surface of the zona pellucida. This is what triggers the acrosome reaction. The acrosome releases the enzyme acrosin, which digests a portion of the zona pellucida. The sperm then fuses with the egg cell’s plasma membrane, as shown in Figure 4, and injects its nucleus and organelles into the egg cell’s cytoplasm.

Figure 4: A diagram showing the acrosome reaction, in which the acrosome at the head of the sperm releases enzymes that digest the zona pellucida, allowing the sperm’s nucleus to enter the egg cell.

This penetration triggers the egg cell, which at this point is still a secondary oocyte, to finally complete meiosis II. The egg cell becomes a mature ovum with a haploid nucleus. The haploid nucleus of the sperm cell then fuses with the haploid nucleus of the ovum, forming a diploid zygote with 46 chromosomes and completing the process of fertilization. If the sperm carried an X sex chromosome, the zygote will have the XX genotype and will develop into a female fetus. If the sperm carried a Y sex chromosome, the zygote will have the XY genotype and will develop into a male fetus.

After the sperm cell has penetrated into the ovum and fertilization has occurred, the zona pellucida (the light-orange layer in Figure 4) changes its structure to become impermeable to other sperm. This prevents a condition called polyspermy, in which an egg cell is fertilized by more than one sperm cell. Zygotes formed in this way have an abnormal number of chromosomes and do not usually survive.

Key Term: Polyspermy

Polyspermy is a condition in which an ovum is fertilized by more than one sperm cell.

Once fertilization has occurred, the resulting zygote undergoes several stages of development, in one of the most remarkable processes in biology, which transforms the zygote into an embryo and then into a fetus. Let’s take a look at these fascinating stages.

As we have learned, fertilization happens in the ampulla of the fallopian tube. Upon fertilization, the zygote begins its journey along the fallopian tube and toward the uterus, pushed forward by ciliary action and muscle contraction. Along the way, the cells of the zygote mitotically divide, as shown in Figure 5. At this stage, each division is referred to as a cleavage. This is the stage at which the zygote becomes an embryo. Each cell within the embryo is called a blastomere. Once the embryo contains 8–32 tightly packed blastomeres, which can take about 4 days, it is called a morula.

Figure 5: A diagram showing the stages of cell division, or cleavage, in a developing embryo before implantation in the uterus.

The morula continues to divide, moves to the uterus, and transforms into a blastocyst by day 6. At this stage, the blastocyst hatches out of the zona pellucida layer that was originally surrounding the egg cell. As you can see in Figure 5, the blastocyst is surrounded by a layer of cells called the trophoblast. Within the trophoblast is a small group of cells called the inner cell mass, as well as a fluid-filled cavity called the blastocoel.

The trophoblast attaches itself to the endometrium, as shown in Figure 6. The endometrial tissue envelops the blastocyst, embedding it into the endometrium. This process is called implantation and occurs between 6 and 12 days after fertilization.

Once implantation has occurred, the inner cell mass can begin to differentiate, eventually developing into the cells that form the organs of the embryo and later on the fetus over the period of pregnancy, called the gestation period.

Key Term: Embryo

The embryo is the early developmental stage of an organism. The zygote undergoes cleavage to become an embryo, which eventually develops into a fetus over the gestation period.

Key Term: Implantation

In pregnancy, implantation is the process by which an embryo attaches to the endometrium and becomes embedded in it.

Figure 6: A diagram showing the stages of cleavage in a developing embryo and the journey from the fallopian tube to the uterus, where implantation happens.

Example 2: Identifying the Stages of Embryo Development

The figure shows the stages of development that occur after fertilization. What is the name of structure X?

  1. Graafian follicle
  2. Morula
  3. Zygote
  4. Fetus
  5. Blastocyst

Answer

As we know, humans reproduce sexually, which means that a human zygote is formed when a male gamete (or a sperm cell) fuses with a female gamete (or an egg cell). This process is called fertilization, and this is what we can see depicted at the beginning of the diagram above.

Once a zygote is formed, the cells within the zygote begin to mitotically divide. This process is called cleavage, and each cell within the zygote at this stage is called a blastomere. In this process, the blastomeres multiply from 2 to 4 and so on. Once the zygote has around 8–32 blastomeres, it forms a tightly packed structure called the morula. The morula then differentiates into a blastocyst, with an outer layer of cells called the trophoblast, a group of cells called the inner cell mass, and a fluid-filled cavity called the blastocoel.

In this question, we are being asked to identify the last structure depicted in the diagram. If we look at the answer options, we can straightaway eliminate morula and zygote, as we know that these structures do not match the one labeled in the diagram.

The Graafian follicle is a fluid-filled structure in the ovary in which the egg cell develops before being released. Since this diagram depicts the events that happen after the release of the egg cell and fertilization, we can eliminate this option.

During the development of the baby within the mother’s uterus, it is called a fetus after about 8 weeks of pregnancy, when the limbs, sex organs, and external structures are formed. We can, therefore, eliminate this option as well.

The correct answer is, therefore, the blastocyst.

Before we begin learning about pregnancy and the stages of development of the embryo, let’s turn our attention to an interesting phenomenon. In some cases, more than one embryo can be implanted in the uterus, leading to multiple births. The most common type of multiple births is the formation of twins. Have you ever seen a set of twins and noticed their similarities? Perhaps you are a twin yourself. Twins may be either identical or nonidentical. But how exactly do twins form, and what decides whether twins will be identical or not?

As we have learned, a zygote is formed when an ovum is fertilized by a sperm cell. When twins are formed, they may develop either from a single zygote or from two separate zygotes.

In some cases, instead of a single ovum being released during ovulation, two ova are released. If each ovum is separately fertilized by a different sperm cell, two zygotes are formed. Since these two zygotes are formed by the fertilization of two different ova by two different sperm cells, they are not genetically identical to one another. If both of these zygotes are successfully implanted and develop into fetuses, nonidentical twins will be born. Nonidentical twins are also called fraternal twins or dizygotic twins, since they develop from two separate zygotes. The word fraternal is used to describe brothers or siblings, implying that fraternal twins have the same level of genetic similarity that normal siblings would have.

Key Term: Dizygotic Twins

Dizygotic twins, also called fraternal or nonidentical twins, are twins that develop from two separate zygotes, where two egg cells are fertilized by two separate sperm cells.

Let’s think about the sex of dizygotic twins. Since they develop from two separate zygotes, they can have either the same sex or different sexes, depending on the sex chromosomes carried by the two sperm cells that fertilized the two ova.

The second type of twins is called monozygotic twins, as they develop from a single zygote. In this case, a single ovum is fertilized by a single sperm, and the resulting zygote splits into two embryos. This may happen between 8 and 12 days after fertilization. In most cases, it is believed that monozygotic twins form because the embryo has split during the early stages of development. Since both embryos come from a single zygote, they are genetically identical to one another, forming identical twins. These twins share all of their genes and characteristics and are, therefore, always of the same sex, as they will both have their genotype decided by the sex chromosome of a single sperm cell.

Key Term: Monozygotic Twins

Monozygotic twins, also called identical twins, are twins that develop from a single zygote that splits into two separate embryos.

In Figure 7, you can see a representation of the formation of dizygotic and monozygotic twins.

Figure 7: A diagram showing the differences between the formation of dizygoti twins and the formation of monozygotic twins.

Let’s now learn about how the embryo develops into a fetus after implantation.

During the second week after the embryo has been implanted, the cells begin to differentiate into layers. Some of these layers form outside the embryo and are called extraembryonic membranes. These layers are crucial for the protection and support of the embryo as it develops into a fetus during the gestation period.

During the second week, a layer called the amnion is formed. A cavity or space called the amniotic sac surrounding the embryo is formed. This cavity becomes filled with amniotic fluid, as shown in Figure 8, and eventually surrounds the entire embryo, behaving as a shock absorber and protecting it from temperature fluctuations, trauma, and sudden, sharp movements. Interestingly, fraternal twins are likely to develop with two separate amniotic sacs.

Key Term: Amnion

The amnion is the innermost extraembryonic membrane that encloses the developing embryo or fetus.

Key Term: Amniotic Fluid

The amniotic fluid is the fluid surrounding the embryo or fetus within the amnion. This fluid protects the fetus from trauma and temperature fluctuations.

Figure 8: A diagram showing a fetus within a womb, surrounded by the amnion. The amnion is filled with amniotic fluid, as depicted in the diagram, which helps protect the fetus from trauma and temperature fluctuations.

Around the 14th day of pregnancy, another extraembryonic membrane called the chorion starts to develop. This forms the outermost layer, which surrounds the entire developing embryo. The chorionic membrane forms finger-like projections called the chorionic villi. Together, these villi and part of the mother’s endometrium form the placenta. Up until this point in pregnancy, the endometrial cells themselves are responsible for nourishing the dividing embryo. However, at this stage, the placenta takes over, supplying the embryo with nutrients and oxygen and carrying away its waste products. Figure 9 shows a diagram of the placenta. Let’s take a closer look at its structure.

Figure 9: A diagram depicting the structure of the human placenta. The umbilical arteries, shown in blue, carry deoxygenated blood away from the fetus, and the umbilical vein, shown in red, carries oxygenated blood toward the fetus.

Key Term: Chorion

The chorion is the outermost extraembryonic membrane that surrounds the developing embryo or fetus.

Key Term: Chorionic Villi

The chorionic villi are extensions from the chorion that associate with the endometrium of the mother’s uterus, forming the placenta.

Key Term: Placenta

The placenta is a temporary organ that supports and nourishes the fetus during its development and secretes the hormones required to maintain the pregnancy.

The placenta is connected to the developing embryo through the umbilical cord, labeled in Figure 9. The umbilical cord is usually 5060 cm long, and it usually contains three blood vessels: two umbilical arteries and one umbilical vein. The umbilical arteries are responsible for transporting deoxygenated blood, which is rich in carbon dioxide and waste products, from the fetus to the placenta. The umbilical vein, on the other hand, supplies oxygenated blood from the placenta to the fetus.

Key Term: Umbilical Cord

The umbilical cord is the cord containing blood vessels that connects the developing fetus to the placenta.

The placenta is an organ where the circulatory systems of the mother and fetus can interact. As you can see in the diagram, the blood vessels of the two never come into direct contact with one another, and their blood does not mix. Instead, the mother’s blood pools in the intervillous spaces in the placenta. Within the chorionic villi, the blood vessels of the fetus form branched, tree-like structures. Across the membranes of the chorionic villi, the mother’s blood can exchange gases and nutrients with the fetal blood vessels through diffusion. The chorionic villi in the placenta serve to increase the surface area for the exchange of materials between the mother and the fetus. It is important to remember that toxic substances, such as alcohol, nicotine, and other harmful drugs, can cross the placenta and reach the developing fetus. This can cause developmental defects and diseases, which is why it is crucial for pregnant women to avoid exposure to such toxic substances.

Example 3: Understanding the Role of the Placenta in the Exchanging Process

The figure shows the structure of the placenta. Which substance is found in high concentrations in blood vessel X?

  1. Carbon dioxide
  2. Oxygen
  3. Glucose
  4. Alcohol
  5. Amino acids

Answer

The placenta is a temporary organ that is formed by the association between part of the mother’s endometrial tissue and the fetal chorionic villi. This organ serves to support and nourish the fetus during the pregnancy or gestation period. The fetus is connected to the placenta through the umbilical cord, which supplies the fetus with oxygen and nutrients and carries away its waste products, such as carbon dioxide. In the figure shown above, the twisted, extended blood vessels at the bottom of the diagram form the umbilical cord.

The umbilical cord usually contains two umbilical arteries and one umbilical vein. In the figure shown, the two umbilical arteries are depicted in blue and the single umbilical vein is depicted in red. The umbilical vein is responsible for carrying oxygenated blood from the placenta to the fetus, whereas the two umbilical arteries carry deoxygenated blood from the fetus to the placenta.

In the figure, the label X points to one of the two blue, twisted blood vessels in the umbilical cord. This is an umbilical artery, which means that this blood vessel carries deoxygenated blood away from the fetus and toward the placenta. Deoxygenated blood is rich in the fetus’s waste products, which include carbon dioxide.

The correct answer is, therefore, carbon dioxide.

Aside from its role in nourishing the fetus, the placenta also has endocrine functions. It secretes hormones that help support and maintain the pregnancy. Once the placenta forms, it begins to secrete a hormone called human chorionic gonadotropin, or hCG. During pregnancy, this hormone is responsible for maintaining the endometrial thickness throughout the pregnancy to support the fetus.

The hormone hCG also ensures that the corpus luteum, which is a temporary endocrine structure in the ovaries, continues to secrete progesterone at the beginning of the pregnancy, without degenerating as it would if fertilization had not occurred. Around the fourth month of pregnancy, the corpus luteum degenerates, and the placenta itself takes over the function of secreting progesterone for the remaining months of pregnancy. Progesterone plays an important role during pregnancy by preventing the uterus from contracting and by increasing the blood flow to the uterus. The placenta also secretes estrogen, which increases vascularization, or the formation of blood vessels, in the uterus, which helps support the developing fetus. The levels of estrogen and progesterone increase steadily throughout the duration of the pregnancy.

Now that we have learned about the placenta and extraembryonic membranes, let’s circle back to our discussion about how twins form. Identical or monozygotic twins, which develop from a single zygote, generally share a single placenta, although the two embryos are connected to the placenta through two separate umbilical cords. Dizygotic or fraternal twins, on the other hand, usually have two separate placentas and umbilical cords.

Example 4: Describing the Characteristics of Monozygotic Twins

What is a characteristic of monozygotic twins?

  1. They are not genetically identical.
  2. They are formed from 1 egg and 2 sperm.
  3. They are always the same sex.
  4. They develop with the same umbilical cord.

Answer

We know that a zygote is formed by the fusion of a male gamete (or a sperm cell) with a female gamete (or an egg cell). This process is called fertilization. To answer this question, let’s first describe monozygotic twins. Usually, when fertilization takes place, a single zygote is formed, which develops into a single fetus. However, in some cases, multiple babies are born from a single pregnancy. Giving birth to twins is the most common form of multiple births.

Monozygotic twins are twins that develop from a single zygote. This means that one zygote is formed by the fusion of one sperm cell with one egg cell, and this zygote then splits into two embryos. If the two embryos are implanted successfully, they develop into monozygotic twins. Using this information, we can eliminate the option that says that they are formed from 1 egg and 2 sperm.

When a fetus develops within the mother’s uterus, it is nourished and supported by a temporary organ called the placenta. The fetus is attached to the placenta through the umbilical cord, which supplies the fetus with oxygen and nutrients and carries away its waste products. When monozygotic twins are formed, they usually share a placenta, but each of them is connected to this placenta through a separate umbilical cord. We can, therefore, eliminate the option that says that they develop with the same umbilical cord.

Let’s now think about the genetics of monozygotic twins. Since monozygotic twins develop from the same egg and sperm cell, they share all of their genes and are genetically identical to one another. In monozygotic twins, the sex of both babies is determined by the sex chromosome carried by a single sperm. This means that monozygotic twins are always of the same sex.

The correct description of monozygotic twins, therefore, is that they are always of the same sex.

The period of gestation or pregnancy varies between different organisms. For example, sheep carry their young for about 152 days, whereas African elephants have a gestation period of about 22 months, which is about 670 days. As we have mentioned, in humans, the gestation period is 9 months, which is about 40 weeks or 280 days long. The gestation period or pregnancy is divided into three trimesters. During each trimester, the fetus gradually grows and its organs and organ systems are developed. Let’s outline the different stages of fetal development during pregnancy.

The first trimester describes the period from week 1 to week 12 of pregnancy. During the first trimester, the heart begins to develop as early as the third week of pregnancy. The four-chambered heart of the fetus becomes developed by week 8, and the fetus’s heartbeats can be detected through ultrasound at around week 5 to week 7. This means that the heart is one of the first organs to develop in a growing fetus. The nervous system then begins to form, starting with the spinal cord. The hands and eyes begin to develop, and the sex organs become fully differentiated. In a male fetus, the testes start to develop around week 5 to 6, and in a female fetus, the ovaries develop at around week 12.

The skeletal system also begins to develop during the first trimester, in a process called skeletal ossification, or the formation of bones. This process begins during embryonic development and can continue well into adulthood, up to the age of about 25, although this varies between individuals.

During the second trimester, which spans from week 13 to week 26 of pregnancy, the sense organs of the fetus begin to develop, and the fetus begins to experience some sensory stimulation. At this trimester, the fetus’s fingers and toes also become clearly defined. The nervous system becomes more advanced during the second trimester, and the first signs of movement are usually detected during this stage.

The third and final trimester, from week 27 until birth, is the longest trimester. During this trimester, extensive brain development occurs and all the internal organ systems become completely developed. Sensory development is completed in this trimester. You can see a summary of the three trimesters and the stages of development of the fetus in Figure 10.

Figure 10: A diagram showing the stages of development from an embryo to a fetus during each of the 9 months of the gastation period or pregnancy.

At the end of the 40-week gestation period, the baby becomes fully developed and ready to be born. The process of birth and delivery, also called parturition, involves several systems of the mother’s body working together, enabling the baby to emerge from the uterus. Let’s understand the events involved in this process.

Key Term: Parturition

Parturition is the process of giving birth.

At 40 weeks, when the baby becomes ready to be born, the first thing that happens is that the amniotic sac ruptures. As we have learned, throughout the course of pregnancy, the levels of estrogen and progesterone rise steadily. At the time of birth, however, the level of progesterone drops rapidly. Over the gestation period, progesterone functions to prevent the uterine muscles from contracting. This sudden drop in the levels of this hormone at the time of childbirth causes the uterus to contract at regular intervals. This process of regular uterine contraction is called labor, which pushes the baby out of the uterus through the cervical canal and out of the vagina.

Key Term: Labor

Labor is the process by which the fetus and placenta emerge from the uterus through uterine contractions.

The high level of estrogen produced by the placenta and the mother’s ovaries throughout the course of pregnancy prepares the uterus for parturition, by increasing the oxytocin sensitivity in the uterus. Oxytocin is a pituitary hormone produced in the hypothalamus and stored in the posterior pituitary gland. At the time of childbirth, the contractions of the uterus and the pressure exerted by the baby cause the uterus and cervix to stretch, which stimulates the release of oxytocin from the posterior pituitary into the bloodstream. Oxytocin also induces the uterus to contract, pushing the baby out.

Another hormone, relaxin, is secreted by the ovaries and placenta. This hormone causes the cervix to soften and expand, allowing the baby to pass through the cervical canal, pushed by the uterine contractions, and come out of the vagina. The baby’s umbilical cord must be severed at birth, and the placenta detaches completely from the uterine wall and is pushed out a few minutes later.

The hormones secreted by the pituitary gland enable the production and secretion of milk from the mother’s mammary glands. The baby can be fed immediately after birth with the first milk, called colostrum, which is very rich in antibodies and, therefore, provides the infant with important nutrients and protects it against infections.

Interestingly, women above 35 years of age are more likely than younger women to experience high-risk pregnancies. The age of the father also plays an important role. Several studies have shown that if the father is advanced in age at the time of conception, the baby may experience health problems.

In Figure 11, you can see a summary of the process of parturition divided into three broad stages.

Figure 11: A diagram showing the process of parturition, or childbirth, divided into three stages.

Let’s now summarize the key points that we have learned about fertilization and pregnancy in this explainer.

Key Points

  • Fertilization is the process of fusion of a male gamete (or sperm cell) with a female gamete (or egg cell) to form a zygote.
  • Fertilization occurs in the fallopian tube within the female reproductive system. The zygote then moves toward the uterus while undergoing mitotic division to form a blastocyst.
  • The blastocyst gets embedded in the endometrium of the uterus, in a process called implantation, where it is supported throughout its growth and development.
  • The developing fetus is nourished through the placenta, which helps exchange materials between the mother and the fetus.
  • Pregnancy is divided into three trimesters, during which the organs and organ systems of the fetus completely develop. During the ninth month of pregnancy, the baby is born, in a process called parturition.

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