Chapter II: How does the pill work ?

Today, there are many different types of contraceptive pills and one is confronted with a variety of choices: the so called mini pills (containing only gestagen ), the combination pills (commonly a combination of estrogen, gestagen and sometimes placebo pills) and others which just add to the decision making hurdles. But do you actually know how exactly does the pill work? To fully understand how the pill functions, it is necessary to get to the basis of the menstrual cycle and the hormones that are involved in it.

The pill works as a contraceptive by preventing the fertilization of an egg by a sperm cell. In order to avoid pregnancy, the pill uses hormonal mechanisms to keep the sperm and the egg apart and to stop the release of an egg (ovulation). For this to work properly, the pill needs to be taken around the same time every day. Researchers found a way to redirect naturally occurring hormones of the menstrual cycle by utilizing synthetic analogues to those hormones. We will explain the combination pill which contains estrogen and progestogen as active ingredients in varying formulations like synthetic estrogens and progestin.

Progestogens are a group of hormones with progesterone as the most important naturally occurring member. The progestogen that is used in the pill is the synthetic progestin such as drospirenone or levonorgestrel. The estrogen contained in the pill is also a synthetic variant, usually ethinylestradiol.

The hypothalamic–pituitary–gonadal (HPG) axis regulates the production and release of the hormones that are involved in the menstrual cycle and necessary for reproduction see Chapter I of the series.

By introducing synthetic variants of hormones to the blood, estrogen and progesterone feedback to the hypothalamus and the pituitary leading to decreased release of LH and FSH. The decrease of FSH levels leads to impaired follicular development. The lack of the mid-cycle LH surge results in missing ovulation. By suppressing the naturally stimulating signals, the pill prevents egg maturation and ovulation.

However, suppression of ovulation is not the only contraceptive effect caused by the pill. If an egg was somehow released, there are additional mechanisms of action to prevent pregnancy. One of these mechanisms is the reduced uterus tissue lining, as a consequence resulting in less chance for a fertilized egg to be implanted in the uterus. Additionally, the progestin in the pill thickens the cervical mucus and makes it inhospitable for the sperm, functioning like a barrier method. This makes it harder for the sperm to travel through the cervix and enter the upper genital tract. Furthermore, the tube mobility is also impaired while under the pill, altering the movement of the egg through the fallopian tube.

In general, the pill suppresses the normal menstrual cycle and replaces it with an artificial 28 days cycle. LH and FSH are being suppressed, and progesterone and estrogen are kept high until the withdrawal of the dose. That is when the levels of hormones in your pill drop. There is also a so-called withdrawal bleeding. This bleeding is similar to a natural period but usually lighter. Menstruation is suppressed as the rest of the cycle and does not occur while taking the pill. The regular pill should not be mistaken for emergency contraception, or Plan B.

When women want to stop the pill, it often comes to a lot of uncertainties about the changes that will happen to the body. Keep an eye out for our next and last Chapter on this series - what happens when one stops to take the pill.

Reference: https://www.cell.com/trends/ecology-evolution/fulltext/S0169-5347(09)00263-8