What is the Birth Control Pill?
By Calla Hindle - June 18, 2026
The birth control pill is an oral contraceptive that is taken daily at the same time each day. Each pill contains synthetic hormones that aim to prevent pregnancy. Typically, women take active (hormone-containing) pills for 21 days, followed by 7 days of inactive pills.[1] During the week of inactive pills, women will experience bleeding. This bleeding is not a real period but rather a withdrawal bleed, which is the body’s response to the temporary pause in hormone reception.[2]
How the Pill Works
The birth control pill works by manipulating your natural hormone cycle to prevent pregnancy. The majority of pills contain synthetic versions of two key female hormones:
Ethinylestradiol (a synthetic form of estrogen)
Progestin (a synthetic form of progesterone). The progestin used in most birth control pills is called levonorgestrel
Here is how the pill interferes with the body’s natural processes:
Suppressing Ovulation
The main mechanism of action in the birth control pill is to suppress ovulation. Levonorgestrel can prevent the ovaries from releasing eggs.[3] It does this by suppressing the hormone surge of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) that causes the release of an egg.[4] If no egg is available, fertilization cannot occur. The pill does not guarantee complete suppression of ovulation, and breakthrough ovulation occurs in up to 10% of cycles, even if the pill is being taken properly.[5]
Thickening Cervical Mucus
In addition to suppressing ovulation, the birth control pill has two secondary mechanisms that help prevent pregnancy. One of these is the thickening of cervical mucus. Cervical mucus naturally changes in response to hormone fluctuation throughout a normal menstrual cycle and is an important indicator of fertility. During the fertile phase, it becomes thin and slippery, making it easier for sperm to travel through the cervix and increasing the chance of fertilization. In contrast, during the non-fertile phase, progesterone thickens and viscosifies mucus, making it difficult for sperm to pass through.[6] By taking the birth control pill daily, a steady level of levonorgestrel keeps the cervical mucus consistently thick, creating a barrier that helps prevent sperm from reaching the egg.[7]
Thinning the Uterine Lining
The other secondary mechanism that the pill uses to prevent pregnancy is the thinning of the endometrium (uterine lining). Normally, estrogen builds up the uterine lining during the fertile part of the cycle, making it thick and nutrient-rich for potential implantation. Progesterone then maintains this lining for the rest of the cycle. If implantation does not occur, hormone levels drop, and the lining thins and eventually sheds during menstruation.
While the body recognizes levonorgestrel as progesterone, it does not have the same effects on the endometrium as natural progesterone. Natural progesterone stabilizes the uterine lining, keeping it thick and nutrient-rich for potential implantation. Levonorgestrel, on the other hand, weakens and thins the uterine lining. It also reduces the endometrium’s response to estrogen, so even though estrogen levels remain mostly normal, the lining does not thicken.[8]
As a result, the uterine lining stays thin and is less able to support the implantation of a fertilized embryo, leading to the death of the embryo, since implantation is essential for the continuation of a pregnancy.
How Effective is the Birth Control Pill?
With perfect use, taking the pill exactly as prescribed at the same time every day, effectiveness is approximately 99%, resulting in 1 in 100 women becoming pregnant. With typical use (accounting for real-life scenarios where pills may be missed or taken inconsistently), the effectiveness drops to 91%, resulting in 9 out of 100 women becoming pregnant.[9]
Are There Risks Associated With the Pill?
While taking standard birth control pills, female hormones (estrogen, testosterone, and progesterone) are nearly flat-lined.[10] These natural hormones are replaced with the synthetic hormones ethinyl estradiol and levonorgestrel,[11] which are associated with the following common side effects:
Acne
Back pain
Breast tenderness
Changes in the usual menstrual period
Decreased interest in sexual activity
Headaches and migraines
Intolerance to contact lenses
Mood changes
Depression[12]
Anxiety
Nausea
Weight gain
Hair loss
Mood: Due to the severe hormonal disruption caused by the pill, some people experience feeling emotionally distant, less attracted to their partner, or less satisfied overall. These hormonal imbalances can also lead to depression, anxiety, and other significant mood changes. The severity of these changes differs from person to person due to hormonal sensitivity.[13]
Testosterone: The chemical structure of levonorgestrel is similar enough to progesterone for the body to recognize it as progesterone. Still, it is actually more closely related to testosterone than to natural progesterone. This structural similarity helps explain why levonorgestrel can produce androgenic side effects, such as hair loss and weight gain, if taken frequently.[14]
Ovulation: It is important to recognize that ovulation is a vital part of female health that extends far beyond pregnancy. Ovulation and the rest of the menstrual cycle are how women make their hormones. Estrogen and progesterone are essential to a woman's long-term health. Hormonal birth control greatly reduces the production of natural female hormones and replaces them with synthetic ones.[15] These synthetic hormones, unlike natural healthy ones, offer side effects rather than benefits.[16]
Estrogen supports bone health, heart health, and helps regulate mood. The pill causes the body’s natural estrogen levels to drop and replaces them with synthetic estrogen, which does not have the same health benefits as natural estrogen. This increases the risk of bone loss, heart disease, and mood changes.[17]
Progesterone supports immune function, reducing the risk of autoimmune disease and inflammation. It can help prevent breast cancer by counteracting estrogen and support the thyroid and brain.[18] Progestins, however, are not progesterone and have been known to cause androgenic side effects such as masculine hair growth, acne, and weight gain, as well as anxiety, depression, and an increased risk of breast cancer.[19]
Additionally, according to Pfizer Canada, an increased risk of the following serious adverse reactions has been associated with the use of levonorgestrel and ethinyl estradiol, including:[20]
Decreased bone density leading to increased risk of bone fractures
Blood clots
Liver tumors
Cerebral hemorrhage
Cerebral thrombosis
Cervical cancer
Breast cancer
Risk of cancer: Levonorgestrel increases the risk of cervical cancer in women before menopause.[21]A study published by the New England Journal of Medicine found that women who use hormonal birth control have a 20% increased risk of developing breast cancer. The level of risk varied by pill type, ranging from no increase to about a 60% higher risk. The study also found that the longer women used these contraceptives, the higher their risk became.[22]
Cervical intraepithelial neoplasia
Gallbladder disease, including gallstones
Hepatocellular carcinomas
Hypertension
Inflammatory bowel disease (Crohn’s Disease, ulcerative colitis)
Mesenteric thrombosis
Myocardial infarction
Pulmonary embolism
Stroke
Transient ischemic attacks
Heart attack
Thrombophlebitis
Venous thrombosis
Are Birth Control Pills Abortifacient?
An abortifacient is a drug or device that causes an early abortion by preventing the attachment of embryos to the uterus (implantation). While products in Canada are not required to state this on their labels, it is possible to determine whether a form of birth control is abortifacient by examining its mechanisms of action.
The main mechanism of action in the birth control pill is to suppress ovulation by suppressing the hormone surge of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) that causes the release of an egg.[23] If no egg is available, fertilization cannot occur. The pill does not guarantee complete suppression of ovulation, and breakthrough ovulation occurs in up to 10% of cycles, even if the pill is being taken properly.[24] Breakthrough ovulation can lead to fertilization.
The third mechanism that the pill uses to prevent pregnancy is the thinning of the endometrium (uterine lining). Levonorgestrel weakens and thins the uterine lining. It also reduces the endometrium’s response to estrogen, so even though estrogen levels remain mostly normal, the lining does not thicken.[25]
As a result, the uterine lining stays thin and is less able to support the implantation of a fertilized embryo, leading to the death of the embryo, since implantation is essential for the continuation of a pregnancy.
As stated by the American College of Pediatricians in 2017: “At fertilization, a human being emerges as a whole, genetically distinct, individuated zygotic living human organism, a member of the species." At this moment, all genetic material is determined, including eye colour, hair colour, and sex.[26]
Many women take hormonal contraceptives for years without any knowledge of this abortifacient effect. Studies of women aged 18 to 49 in the United States and across 6 European countries found that roughly 80% are unaware that hormonal birth control has the potential to cause an early abortion.[27] Around 33% of women say they would avoid using such birth control methods due to their abortifacient effect.[28]
If the use of abortifacients goes against your moral beliefs or makes you uncomfortable, it is strongly encouraged to find alternative methods. For more information on birth control and fertility awareness, visit naturalwomanhood.org
[1]Felix Health. (n.d.). How do birth control pills work?https://www.felixforyou.ca/blog-posts/what-makes-birth-control-pills-effective#combination-pill
[2]Briden, L. (n.d.). Why pill bleeds are not periods. https://www.larabriden.com/why-pill-bleeds-are-not-periods/
[3]Seidman, L., Kroll, R., Howard, B., Ricciotti, N., Hsieh, J., & Weiss, H. (2015). Ovulatory effects of three oral contraceptive regimens: A randomized, open-label, descriptive trial. Contraception, 91(6), 495–502. https://doi.org/10.1016/j.contraception.2015.03.001
[4] Murray, C. M., & Orr, C. J. (2026). Hormonal regulation of the menstrual cycle and ovulation. In Maternal‑Fetal and Neonatal Endocrinology: Physiology, pathophysiology, and clinical management (2nd ed., pp. 171–180). Elsevier. https://doi.org/10.1016/B978-0-443-23940-3.00031-4
[5]American Academy of Family Physicians. (2001, May 1). Oral Contraceptives and Prevention of Implantation. American Family Physician61(9), 2605–2606.Access article
[6]Gemzell-Danielsson, K., Berger, C., & Lalitkumar, P. G. L. (2018). Mechanisms of action of oral contraceptives. Contraception, 98(1), 8–14. https://doi.org/10.1016/j.contraception.2017.12.004
[7]Gemzell-Danielsson, K., Apter, D., Hauck, B., Rosen, K., Zurth, C., & Uddin, A. (2018). Pharmacokinetics and pharmacodynamics of oral contraceptives. Contraception, 97(5), 413–419. https://doi.org/10.1016/j.contraception.2017.12.004
[8]Beatty, M. N., and P. D. Blumenthal. (2009). The levonorgestrel‑releasing intrauterine system: Safety, efficacy, and patient acceptability. Therapeutics and Clinical Risk Management5, 561–574. https://doi.org/10.2147/tcrm.s5624
[9]WebMD. (n.d.). Birth control effectiveness chart.https://www.webmd.com/sex/birth-control/birth-control-effectiveness-chart
[10]Briden, L. (n.d.). How does birth control affect hormones?[A1]
[11] Drugs.com. (n.d.). Ethinyl estradiol and levonorgestrel: Uses, side effects & warnings.[A2]
[12]Skovlund, C. W., Mørch, L. S., Kessing, L. V., & Lidegaard, Ø. (2016). Association of hormonal contraception with depression.JAMA Psychiatry, 73(11), 1154–1162.https://jamanetwork.com/journals/jamapsychiatry/fullarticle/2552796
[13]American Psychological Association.(2025, April/May). Birth control and the brain.Monitor on Psychology. https://www.apa.org/monitor/2025/04-05/birth-control-brain
[14]Briden, L. (2026, March 20). The crucial difference between progesterone and progestins. https://www.larabriden.com/the-crucial-difference-between-progesterone-and-progestins/
[15]Seidman, L., Kroll, R., Howard, B., Ricciotti, N., Hsieh, J., & Weiss, H. (2015). Ovulatory effects of three oral contraceptive regimens: a randomized, open-label, descriptive trial. Contraception.[A3]
[16]Drugs.com. (n.d.). Ethinyl estradiol and levonorgestrel: Uses, side effects & warnings.https://www.drugs.com/mtm/ethinyl-estradiol-and-levonorgestrel.html
[17]Verywell Health. (n.d.). Estrogen: What does it do for the body?https://www.verywellhealth.com/what-is-estrogen-and-what-does-it-do-to-my-body-4142677
[18]Briden, L. (n.d.). The “superpowers” and benefits of progesterone. https://www.larabriden.com/superpowers-benefits-progesterone/
[19]Drugs.com. (n.d.). Levonorgestrel side effects: Common, severe, long term. https://www.drugs.com/sfx/levonorgestrel-side-effects.html
[20]Health Canada. (n.d.). Product monograph: Alesse 21 and Alesse 28 (levonorgestrel and ethinyl estradiol tablets). https://pdf.hres.ca/dpd_pm/00074282.PDF
[21]Smith, J. S., et al. (2003). Cervical Cancer and use of hormonal contraceptives: A systematic review. The Lancet361(9364), 1159–1167.
[22]Mørch, L.S., et al. (2017). Contemporary hormonal contraception and the risk of breast cancer. New England Journal of Medicine, 377(23), 2228–2239, https://doi.org/10.1056/NEJMoa1700732
[23] Murray, C. M., & Orr, C. J. (2026). Hormonal regulation of the menstrual cycle and ovulation. In Maternal‑fetal and neonatal endocrinology: Physiology, pathophysiology, and clinical management (2nd ed., pp. 171–180). Elsevier. https://doi.org/10.1016/B978-0-443-23940-3.00031-4
[24]American Academy of Family Physicians. (2001, May 1). Oral contraceptives and prevention of implantation. American Family Physician,61(9) 2605–2606.Access article
[25]Beatty, M. N., & Blumenthal P.D. (2009). The levonorgestrel‑releasing intrauterine system: Safety, efficacy, and patient acceptability. Therapeutics and Clinical Risk Management5, 561–574. https://doi.org/10.2147/tcrm.s5624
[26]American College of Pediatricians. (2017, March). When human life begins. https://acpeds.org/position-statements/when-human-life-begins
[27]Migeon, G. (2015, January 23). Women want to know: Does the pill cause abortion? Natural Womanhood.,https://naturalwomanhood.org/women-want-to-know-does-the-pill-cause-abortion/
[28]Dye, H.M., et al. (2005). Women and postfertilization effects of birth control: Consistency of beliefs, intentions and reported use. BMC Women’s Health5(11).https://pmc.ncbi.nlm.nih.gov/articles/PMC1325031/