How the Oral Contraceptive Pill Actually Works

Disclaimer: This article was originally written in 2022 for my Medium blog. The information and research presented reflect the knowledge available at that time. While the content remains relevant, readers should note that newer studies may provide additional insights into the topics discussed here.*

The oral contraceptive pill is the most common form of birth control prescribed in the US. Approximately 25% of 15–40-year-old women use ‘the pill’ as their first choice of contraception. Yet, many young women are being prescribed oral contraceptives without an understanding of how they work.

I was 14 years old when I was first prescribed the contraceptive pill. This didn’t worry me at the time. If it was going to help my painful periods, I was in. However, no one explained to me how it would alter my hormone levels. I didn’t even have a basic understanding of what happened during a menstrual cycle.

It’s important to have an understanding of any medications that you take. Learning about your medications can bring up some questions to ask your doctor. It might even help you understand any side effects you may experience.

What Is the Oral Contraceptive Pill?

There are three types of oral contraceptive pills:

• Combined hormonal pill
• Progesterone only pill
• Continuous or extended-use pill

The combined hormonal pill contains estrogen and progesterone. These are hormones that your body naturally creates in the ovaries and adrenal glands. Fat cells also produce a small amount of estrogen.

The contraceptive pill is mostly prescribed to prevent pregnancy. Although, about 14% of prescriptions are for non-contraceptive reasons. These include:

• Acne
• Endometriosis-related pain
• Fibroids
• Irregular periods
• Menstrual pain
• Menstrual-related migraines

To understand how the pill works, you first need to know what happens in the menstrual cycle. The following paragraphs describe what happens during a typical menstrual cycle. The exact timing varies for each person. Some conditions, such as polycystic ovary syndrome (PCOS) and endometriosis, may cause irregularities.

Overview of the Menstrual Cycle

Menstrual cycles can be 20–35 days long, with the average being 28 days. The menstrual cycle begins on the first day of menstruation. So, the day you start your period is day 1 of your cycle.

Ovulation occurs halfway through your menstrual cycle, typically on day 14. This means that during a typical menstrual cycle, you have 14 days pre-ovulation and 14 days post-ovulation. Ovulation is the process in which the oocyte — the immature egg — is released from the ovary.

The menstrual cycle involves two synchronised processes: the ovarian cycle and the uterine — or endometrial — cycle.

The ovarian cycle occurs in the ovaries. The ovarian follicles develop, ovulation occurs, and estrogen and progesterone are produced.

The uterine cycle occurs in the uterus. This cycle involves the growth, thickening, and shedding of the endometrium.

The endometrium is the tissue that lines the inside of the uterus. It is made up of two layers: the functional layer and the thin basal layer. The functional layer is shed during menstruation, resulting in the menstrual period. The basal layer feeds the functional layer while thickening to prepare for the implantation of a fertilised gamete. A fertilised gamete is the result of a successful fusion of an egg and sperm.

The ovarian cycle has two phases: the follicular phase and the luteal phase. The uterine cycle also has two phases: the menstrual and proliferative phase and the secretory phase.

The ovarian follicular phase and the uterine menstrual and proliferative phase occur during the first 14 days of the menstrual cycle (pre-ovulation). The ovarian luteal and uterine secretory phases occur during days 15–28 (post-ovulation).

The Ovarian Follicular Phase (Days 1–14)

The hypothalamus and anterior pituitary are parts of the brain that are crucial to the menstrual cycle. Pre-puberty, the hypothalamus secretes gonadotropin-releasing hormone (GnRH) at a constant rate. But, after puberty, the hypothalamus releases GnRH in pulses.

When the hypothalamus secretes GnRH, it causes the anterior pituitary to secrete follicle-stimulating hormone (FSH) and luteinising hormone (LH). These two hormones control the maturation of the ovarian follicle.

The immature ovarian follicle is made up of 3 main components

1. The primary oocyte
2. Granulosa cells
3. Theca cells

Granulosa cells and theca cells secrete hormones as the follicle develops.

During the follicular phase, the ovarian follicles compete to grow the fastest.

During the first 10 days of the follicular phase, the ovaries begin to produce estrogen. The theca cells in the follicle bind LH via molecules called receptors. This causes the theca cell to release a hormone called androstenedione.

Simultaneously, the granulosa cells of the follicle bind FSH and subsequently release aromatase. Aromatase is an enzyme that catalyses the conversion of androstenedione to 17𝛽-estradiol (estrogen).

During days 10–14 of the menstrual cycle, the granulosa cells develop receptors for LH.

As the follicle grows, more estrogen is released from the ovary into the bloodstream. Estrogen acts as a negative feedback signal by reducing the amount of FSH being released from the anterior pituitary.

Because of the decline in available FSH, some ovarian follicles will stop growing and die off. The follicle that has the most FSH receptors can bind the most FSH and will continue to grow. This becomes the dominant follicle.

The dominant follicle continues to secrete estrogen. The estrogen makes the pituitary more responsive to the GnRH from the hypothalamus. This causes the pituitary to release more LH, causing a ‘surge’. This surge typically occurs 1 or 2 days before ovulation.

As a result of this LH surge, the ovarian follicle ruptures, releasing the oocyte. This is ovulation.

The Uterine Menstrual and Proliferative Phase (Days 1–14)

The menstrual phase (days 1–5). The menstrual phase is what you would call your period. The menstrual cycle begins on the first day of your period which typically lasts 5 days but it can vary. The bleeding you experience during your period is the shedding of the functional layer of the endometrium.

The proliferative phase (days 6–14). Due to the increase in estrogen — caused by the processes occurring in the ovary — three important changes occur in the uterus:

1. The endometrium thickens
2. Endometrial glands grow
3. Spiral arteries emerge

Spiral arteries supply blood to the functional layer of the endometrium while it thickens. The endometrial glands secrete nutrients to feed the functional layer. This is to prepare your uterus for reproduction.

The increased amount of estrogen alters the thickness of the mucus in the cervix, making it more hospitable for sperm. All the changes that occur in the proliferative phase are to optimize the chance of fertilization.

The Ovarian Luteal Phase (Days 15–28)

After ovulation, the remnant of the ovarian follicle becomes the corpus luteum. The theca and granulosa cells are now ‘luteinised’ as they both bind LH.

The luteinised theca cells continue to produce androstenedione. Androstenedione is then converted to estrogen by aromatase as in the follicular phase.

The luteinised granulosa cells also increase the activity of an enzyme called P450scc. This enzyme converts cholesterol in the body to pregnenolone — the precursor of progesterone.

The granulosa cells produce more progesterone than estrogen during the luteal phase. This means progesterone is the dominant hormone in this phase.

The presence of progesterone causes the pituitary to decrease the amount of LH and FSH being released. Additionally, the granulosa cells begin to secrete inhibin. Inhibin is a protein that also decreases the synthesis and release of FSH from the pituitary.

Towards the end of the luteal phase, the corpus luteum degenerates and becomes the corpus albicans. The corpus albicans does not produce hormones. This causes a decrease in both estrogen and progesterone.

Uterine Secretory Phase (Days 15–28)

The increase in progesterone at the beginning of the uterine luteal phase signals that ovulation has occurred. This causes the endometrium to be more receptive to a fertilized gamete.

The spiral arteries in the endometrium continue to grow and begin to coil. The uterine glands begin to secrete more mucus and nutrients.

From day 15, the window for fertilisation starts to close. This means you are less likely to become pregnant in the days following ovulation — although it is still possible! This is because the cervical mucus starts to thicken, which is harder for sperm to get through.

When the levels of progesterone in the body get very low, the spiral arteries in the endometrium collapse. The functional layer of the endometrium sloughs off, resulting in the menstrual period.

Here’s a graph of the hormonal changes that occur during a typical menstrual cycle.

How the Oral Contraceptive Pill Affects the Menstrual Cycle

Whether the contraceptive pill you take is a combination pill or progesterone only, the main effects are due to progesterone. When you take the pill, you are putting progesterone into your body for the entirety of the cycle, not just in the second half as your body would naturally produce it.

The presence of progesterone alters the follicular phase of the menstrual cycle. It prevents follicular development and ovulation. Essentially, your ovaries do not go through the follicular phase when you take the contraceptive pill.

Progesterone decreases the pulse frequency of GnRH release from the hypothalamus. Because the pituitary releases FSH and LH in response to GnRH, the secretion of FSH and LH is also reduced.

As a result, the ovarian follicle does not develop and does not produce estrogen. Since no estrogen is being released, it can not increase the pituitary’s responsiveness to GnRH. This suppresses the release of LH and inhibits the LH surge.

When there is no LH surge, there will be no follicle rupture and oocyte release. This means that ovulation does not occur.

Here’s a graph of the hormonal changes that occur during a menstrual cycle when taking an oral contraceptive pill.

Here’s a comparison of your hormones during a typical menstrual cycle and your hormones when taking a contraceptive pill.

Conclusion

There are many benefits to taking an oral contraceptive pill, but, it is important to have an understanding of how it works.

If you are on the contraceptive pill or are considering starting it, I hope this article helped you understand the changes that it causes in your body.