Inconceivable! The Mechanisms Behind Birth Control and the Importance of Knowing What is Best for You

By Gaelyn Lyons

Ever since the Supreme Court overturned Roe V. Wade in July2022, there have been concerns about the right to birth control for people who menstruate. Justice Clarence Thomas’s comments about reconsidering previous Supreme Court decisions on birth control fueled many of these concerns.1 In response, the House of Representatives passed a bill that would protect individual’s rights to access and use contraception and protect health care providers’ abilities to prescribe contraception.2,3 This bill now awaits a decision from the Senate.

A significant disagreement between those for and against using birth control comes down to their belief of when pregnancy begins and whether birth control is considered an abortifacient. Depending on your beliefs, an abortifacient can mean two different things. The American College of Obstetrics and Gynecology defines an abortifacient as a substance meant to terminate a pregnancy to prevent live birth.4 However, other organizations define an abortifacient as anything that prevents and terminates pregnancy before viability.5 These differing definitions play a major role in the decisions that are made both at a political and personal level, so it is important to understand the biology behind the menstrual cycle and various birth control methods.

The Menstrual Cycle

The menstrual cycle is a tightly controlled reproductive process for individuals who have periods.6 The average cycle occurs over 28 days, with four phases dictated by the release of hormones, including estrogen and progesterone (Figure 1).7 Day one of the cycle starts the menstrual phase. During this phase, the uterus sheds the thickened lining that was built up throughout the other phases and moves out of the body through the vagina.8 The menstrual fluid that is released contains blood, cells of the uterine lining, and mucus.

Figure 1: The menstrual cycle is separated into two overall phases: the follicular phase, which includes the menstruation phase and ovulation, and the luteal phase, which involves the increase of progesterone and the formation of the corpus luteum. (

The follicular phase includes the menstrual phase and proliferative phase, and occurs on day one of the menstrual phase, lasting until ovulation.8 During the follicular phase, the pituitary gland in the brain releases follicle stimulating hormone (FSH), which signals to the ovaries to begin forming a mature egg in tiny nodules called follicles (Figure 2).8 These forming follicles cause an increase in the hormone estrogen. During this time, the uterus begins to form a thickened lining as the cells proliferate.8

Figure 2: Egg development in the ovary. Follicles form an  egg until the egg is released and moves toward the uterus. The ruptured follicle forms the corpus luteum during the luteal phase.

As estrogen levels increases, the hypothalamus releases another hormone called gonadotrophin-releasing hormone (GnRH), which signals the increase of luteinizing hormone (LH) and FSH.8 The high level of LH initiates the ovulation phase, causing the release of the mature egg from the follicle, and allowing the egg to travel to the uterus.8

In the final luteal phase, the ruptured follicle on the ovary changes into a structure called the corpus luteum.8 The corpus luteum releases progesterone and low levels of estrogen, which supports the thickening of the uterine lining.8 If implantation occurs, which is when the fertilized egg attaches to the uterine lining, the hormone human chorionic gonadotropin (HCG) supports the corpus luteum to continue producing progesterone to sustain a hospitable environment for the implanted egg. 8 However, if there is no implantation, the corpus luteum begins to wither and die, leading to a drop in progesterone levels and the shedding of the uterine lining, beginning the new cycle with menstruation.8

How Hormonal Contraception Works

The menstrual cycle can be affected by using hormonal contraception. Hormonal contraception works by manipulating different parts of the menstrual cycle using either a combination of estrogens and progestins (combined oral contraception, COC) or just progestin.9 While both types of hormonal birth control can stop ovulation, thicken the mucus in the cervix, and prevent the thickening of the uterine lining, their main mechanisms of birth control differ. 9,10  COC prevents ovulation by inhibiting FSH whereas progestin-only birth control thickens the cervical mucus to prevent sperm from getting to the egg.11 There are many ways in which hormonal contraception can be delivered, including oral pills, injections, vaginal rings, implants, patches, and intrauterine devices (IUDs).12

Notably, hormonal contraceptives are not only used for birth control, but also used for management of gynecological issues like endometriosis and polycystic ovarian syndrome, and other medical conditions like acne, migraines, and anemia.9,13 Regardless, using hormonal contraception for birth control is just as important as any of these medical uses, especially for individuals with conditions that can put them into high-risk pregnancies.9

Other Types of Birth Control

Hormonal contraception isn’t the only form of birth control. Non-hormonal IUDs contain copper that causes a safe inflammatory response that is toxic to the egg and sperm.14 Other forms of non-hormonal birth control include single-use options like condoms, diaphragms, and cervical caps (Figure 3).12 Additional forms of birth control that are more permanent include female tubal ligations and male vasectomies. Tubal ligations involve cutting or tying off the fallopian tubes to prevent the egg and the sperm from joinng,15 while vasectomies involve cutting the tubes that carry the sperm.16 For people that are settled on their choice not to have children, these permanent types of birth control are popular. In fact, many doctors are seeing a dramatic increase in the amount of males without children inquiring and getting vasectomies since the overturning of Roe.17

Figure 3: Multiple methods of contraception (

Learning and understanding how birth control works and can be delivered is important to make informed health decisions. In addition, scientists and medical professionals need to clearly define when pregnancy occurs so that individuals can further make health decisions for themselves and decide if birth control is right for them.


  • The right to birth control could potentially change after Roe v. Wade was overturned in July 2022.
  • Depending on your beliefs, the definition of abortifacient could mean different things.
  • There are four phases of the menstrual cycle that can be regulated by hormonal birth control.


1.        Supreme Court abortion ruling has birth control advocates worried : NPR.

2.        House Passes Birth Control Bill Under Cloud of Abortion Politics.

3.        Manning, K. E. Text – H.R.8373 – 117th Congress (2021-2022): Right to Contraception Act. (2022).

4.        reVITALize: Gynecology Data Definitions | ACOG.

5.        Horner, C. & Campo-Engelstein, L. Dueling Definitions of Abortifacient: How Cultural, Political, and Religious Values Affect Language in the Contraception Debate. Hastings Cent. Rep. 50, 14–19 (2020).

6.        Mihm, M., Gangooly, S. & Muttukrishna, S. The normal menstrual cycle in women. Anim. Reprod. Sci. 124, 229–236 (2011).

7.        Normal Menstruation (Monthly Period): Menstrual Cycle & Symptoms.

8.        Menstrual cycle – Better Health Channel.

9.        Schrager, S., Larson, M., Carlson, J., Ledford, K. & Ehrenthal, D. B. Beyond Birth Control: Noncontraceptive Benefits of Hormonal Methods and Their Key Role in the General Medical Care of Women. 29, 937–943 (2020).

10.      Rivera, R., Yacobson, I. & Grimes, D. The mechanism of action of hormonal contraceptives and intrauterine contraceptive devices. Am. J. Obstet. Gynecol. 181, 1263–1269 (1999).

11.      Contraceptive pills | The Royal Women’s Hospital.

12.      Birth control methods | Office on Women’s Health.

13.      Noncontraceptive Benefits of Birth Control Pills.

14.      Copper IUD (ParaGard) – Mayo Clinic.

15.      Tubal ligation – Mayo Clinic.

16.      Vasectomy – Mayo Clinic.

17.      Vasectomies Among the Young and Childless May be on the Rise – The New York Times.

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