Lesson Explainer: Contraception Biology • Third Year of Secondary School

In this explainer, we will learn how to explain how hormones are used in contraception and evaluate the effectiveness of hormonal and nonhormonal methods of contraception.

Did you know that evidence has shown that women who take hormonal contraceptives, such as the birth control pill, can reduce their risk of developing ovarian cancer by up to ? However, these contraceptives can have other serious risks and side effects. Some evidence suggests, for example, that they can increase the risk of developing breast cancer and cervical cancer. For some people, the benefits of these contraceptives in preventing pregnancy outweigh the risks they are sometimes associated with.

Let’s see how pregnancy is caused before we look into the methods of birth control that can prevent it.

Pregnancy is a result of an egg cell (or ovum) and a sperm cell fusing, which usually results from sexual intercourse between a male and a female. A sperm is a male sex cell, or gamete, that contains half of the typical genetic material of a normal body cell. Cells that contain half the genetic material are called haploid cells.

An egg cell is the female sex cell, or gamete. Egg cells also contain half the genetic material that would be found in a normal body cell and so are called haploid.

You can see the main structures in the female reproductive system that are involved in fertilization, the process by which the egg cell and sperm cell fuse together, in Figure 1 below.

In Figure 1, the journey of the egg cell is shown in yellow and the journey of the sperm cell in blue. Note that the sizes of these two cells are shown much larger than they actually are!

During sexual intercourse, sperm enters the vagina and travels through the cervix and into the uterus. From the uterus, sperm may enter the fallopian tubes, sometimes called oviducts. If the female is ovulating, an egg cell is released from one of her ovaries into one of the two fallopian tubes. Keep in mind that while there is usually only one egg cell released during each ovulation, millions of sperm cells can be released during sexual intercourse. Figure 1 shows the journey of just one of these sperm cells.

Fertilization is most likely to occur if a sperm cell from a male meets this egg cell from the female and the two combine with each other. This forms a zygote that can implant into the wall of the uterus if conditions are suitable, and the woman becomes pregnant.

Let’s look at how contraceptives can prevent fertilization and pregnancy from occurring.

Contraception is a method by which fertilization, and therefore pregnancy, can be prevented. The word contraception comes from a shortened version of the word conception meaning “to conceive,” which usually refers to conceiving a child through fertilization. Contra means “against,” as contraceptives work against this conception occurring.

There are generally two different methods of contraception: the hormonal method and the barrier method.

Let’s look at some examples of the hormonal method of contraception.

Hormonal contraceptive methods usually use low doses of artificial estrogen, progesterone, or both to prevent pregnancy.

One of the most commonly used methods of hormonal birth control is a tablet called the contraceptive pill (otherwise known as the birth control pill, or sometimes just “the pill”) that is taken orally. There are different forms of the contraceptive pill that release different artificial hormones when they are broken down in the digestive system, but here we will focus on the combined pill.

The combined pill contains artificial versions of the hormones estrogen and progesterone. These hormones are released when the pill is broken down in the digestive system and can then be absorbed into the bloodstream. Once the hormones are in the blood, they travel to various target organs, especially the female reproductive organs.

You can see how these hormones might affect the female reproductive system in Figure 2 below.

One such effect of taking the contraceptive pill may be stopping ovulation. Remember, ovulation is when an egg cell is released from the ovary. The prevention of ovulation is represented in Figure 2 by the red cross through the yellow arrow that would otherwise show an egg cell being released from the ovary. If egg cells are not released, they cannot be fertilized by a sperm cell.

Another effect of hormonal methods of contraception may be thickening the cervical mucus, represented by the purple oval in the cervix in Figure 2. When cervical mucus thickens, sperm cannot pass through the cervix to enter the uterus and therefore the fallopian tubes, so they cannot fertilize an egg cell.

There are several other hormonal methods of contraception, some of which you can see in Figure 3 below.

Let’s look at a few of these methods in a little more detail.

One method is the contraceptive injection. The injection delivers artificial hormones directly into the bloodstream and must be administered by a medical professional every one to three months, depending on the type of injection.

The contraceptive implant is a small plastic device that is implanted under the skin of the upper arm. There, it releases artificial hormones, such as artificial progesterone, directly into the bloodstream. This can last up to three years. Similarly to the injection, this must be administered by, and eventually removed by, a medical professional in a medical setting.

The hormonal IUD, or intrauterine device, sometimes known as the coil, is another hormonal method of contraception that must be administered and removed by a medical professional. Hormonal IUDs are made of plastic, and, depending on the type, can come in all shapes and sizes and continue to be effective for between three and seven years. The most common hormonal IUDs are shaped like a “T” as you can see in Figure 3. The hormonal IUD is implanted into the uterus where it releases artificial progesterone into the bloodstream.

It is worth noting that there is also a copper version of the IUD that does not involve releasing artificial hormones. The copper IUD is also inserted into the lumen of the uterus and looks very similar to the hormonal IUD with the exception that it is made from plastic and copper. Copper causes an inflammatory response in the uterus which acts as a spermicide. This disables the movement of sperm to prevent them from reaching and fertilizing an egg cell. In the unlikely occurrence that the egg cell is fertilized, the physical presence of the IUD usually prevents a fertilized egg cell from implanting in the uterine wall.

Effectiveness, in terms of contraception, refers to how successful a certain contraceptive method is at preventing pregnancy. Effectiveness is usually expressed as a percent of the average number of women who will not become unexpectedly pregnant when using a specific contraceptive.

In this explainer, we will be looking at the comparative effectiveness of the different methods of contraception. All these percentages have been taken from the United Kingdom’s National Health Service (NHS) in 2020, so it is important to note that these values may differ slightly between countries and years.

There are many different methods of hormonal contraception which all differ slightly in their effectiveness. Hormonal contraceptives are, however, all more than effective at preventing pregnancy, if they are used properly.

This means that if 100 women used the combined pill as their only method of birth control, up to 99 of these women would be unlikely to have an unexpected pregnancy within a year, but 1 of them will. You can see this by looking at the diagram in Figure 4. This outcome will only occur when no mistakes are made and the pill is taken with “perfect use.”

Humans do make mistakes, so what errors can cause the pill to not have its intended effect of preventing pregnancy?

Some contraceptive pills must be taken every day, and if the pill is forgotten one day, it may not be working to prevent pregnancy. For the most commonly used contraceptive pill, the time this pill is taken must be within the same hour every day. The risk of user error with devices such as the injection, the implant, or the IUD is much lower as they are administered by a medical professional.

Contraceptive methods become less effective with “typical use.” “Typical use” describes when the method is not always used correctly, for example, forgetting to take a pill one day. With “typical use,” the combined pill is around effective at preventing pregnancy. This means that out of 100 women using the combined pill, 9 of them are expected to become pregnant in a year, as you can see in Figure 5.

It is important to note that hormonal methods of birth control do not prevent transference of sexually transmitted infections (STIs), such as chlamydia, HIV, and syphilis. This is because hormonal methods of contraception do not block the transmission of bodily fluids from one person to another. Bodily fluids, such as blood, semen, or vaginal discharge, can carry disease-causing microorganisms in a person infected with an STI.

When these microorganisms are transmitted via fluid in sexual intercourse from one person to another, the STI can also be transmitted to the other person. To block the transmission of STIs, a barrier method of contraception must be used.

Let’s look at the barrier method of contraception now.

Barrier methods prevent any sperm from reaching the egg cell, therefore preventing fertilization and a subsequent pregnancy. Some of these methods are also beneficial at preventing the transmission of STIs. This cannot be achieved by hormonal contraception, which only functions to prevent pregnancy.

Condoms are one of the most commonly used barrier methods of contraception. A male condom is a thin layer which is usually made of latex that fits over the male’s penis. Condoms can also be made in other materials as some people are allergic to latex, though these other materials are usually less effective at preventing pregnancy. Female condoms are an alternative method that fit inside a female’s vagina. Regardless of which condom is used, it must be applied before intercourse and used for the full duration of sexual intercourse to be effective at preventing pregnancy and transmission of STIs.

If they are used properly, both male and female condoms work by forming a physical barrier. This means that it is much harder for a sperm cell to reach the egg cell as the egg cell travels from the ovary through the fallopian tubes. It is therefore unlikely for a sperm to fertilize the egg cell, preventing pregnancy.

If used properly, this barrier method also means that bodily fluids are less likely to be exchanged, so transmission of STIs is less likely. The female condom covers some of the external genitalia, further reducing the likelihood of transmission of STIs.

Just as hormonal contraceptives run the risk of failing due to their users’ error, condoms can tear or break or be applied incorrectly.

With “perfect use,” male condoms are about effective at preventing pregnancy, but with “typical use” they are only effective. This means that 18 women out of 100 are likely to become pregnant within a year when using male condoms as their only method of birth control, as you can see in Figure 6.

These values are slightly lower for female condoms. With “perfect use,” they are effective, and with “typical use” they are around effective. This means that around 21 women out of 100 are likely to become pregnant within a year when using female condoms as their only method of birth control.

There are other barrier methods of contraception which can help prevent pregnancy but do not prevent the transmission of STIs. One such method is the contraceptive diaphragm, which is often a latex dome that fits over the cervix. Sometimes the contraceptive diaphragm can be made of other materials, such as silicone. It must be used in combination with spermicidal gel which kills or disables the movement of any sperm that reach it. This prevents the sperm from swimming through the cervix, so they cannot pass into the fallopian tubes to reach and fertilize an egg cell. As it does not stop bodily fluids from being exchanged, the diaphragm does not prevent the transmission of STIs.

Another nonhormonal contraceptive method involves surgical procedures called sterilization, which you can see in Figure 7 below. These procedures are performed by a doctor and are usually permanent and difficult to reverse.

In males, sterilization is called a vasectomy and involves cutting and tying shut the vas deferens, the tube that carries sperm from the testes. This prevents sperm from entering the semen that is ejaculated from the penis and therefore prevents sperm cells from entering the female’s vagina. If the sperm does not enter the vagina, it cannot fertilize an egg cell.

In females, sterilization is called tubal ligation and involves cutting and tying shut the fallopian tubes. This prevents the egg cell from passing through the fallopian tube so it cannot be fertilized by a sperm cell.

It is important to note that the most certain method of preventing a pregnancy is by abstaining from sexual intercourse.

Some methods that are sometimes used are significantly less effective than those we have described in this explainer. For example, the “rhythm method” involves abstaining from sexual intercourse only when a female ovulates. This method is not effective at preventing pregnancy for a number of reasons. Primarily, it relies on the menstrual cycle being perfectly regular to ensure that predictions about ovulation are accurate, but the menstrual cycle is not perfectly regular. Furthermore, sperm cells can survive in the reproductive tract for up to five days after sexual intercourse. This means that if a person engaged in sexual intercourse up to five days prior to ovulation and then abstained from sex, they could still become pregnant.

Table 1 below compares the effectiveness of using some hormonal and barrier methods of contraception with “perfect use” and “typical use.”

The typical use for the contraceptive implant and that for the IUD are not applicable as they are administered by a medical professional and are much less likely to be subject to user error. Typical use for the sterilization method is also not applicable as it tends to be very effective and usually irreversible.

To reduce the chance of pregnancy further, some people choose to use a combination of hormonal and barrier methods. It is important to recall that only barrier methods can prevent STIs, and even barrier methods are not effective at preventing STI transmission. The most certain method of preventing pregnancy or transmission of STIs is abstaining from sexual intercourse and any activity that includes transmission of bodily fluids between different people.

Let’s recap some of the key points we have covered in this explainer.