After Roe v. Wade was overturned this past summer, I saw a spark of online discussions about access to birth control, as well as misinformation online on how different contraceptive methods work. Plan B has many misconceptions surrounding it, and seeing as the misuse of the pill can result in life-altering (and life-creating) consequences, it’s critical that people who use it understand how it works.

Plan B is known as a “morning after” pill to prevent pregnancy after unprotected intercourse or failed contraceptive methods (which would be Plan A). As opposed to abortion pills (that we can think of as Plan C), which terminate a preganancy that has already started, Plan B prevents a pregnancy from beginning by preventing the process of ovulation, fertilization, and implantation that causes a pregnancy. Compared to other forms of birth control pills, Plan B is only used as an emergency treatment after intercourse instead of being taken regularly, and is available over the counter instead of by prescription.

An Overview of How Pregnancy Occurs

Before talking about how Plan B prevents a pregnancy, we have to cover how pregnancy actually happens. Let’s put on our scientist thinking-caps and see if we can notice any details about the process that might be helpful to develop a way of preventing it.

Ovulation is when a mature egg is released from an ovary into a fallopian tube where the egg moves towards the uterus. The egg will move around the fallopian tube for 12-24 hours. Also during ovulation, the uterus is prepared for a potential pregnancy by the inner layer becoming spongy and thick, the ideal environment for a fertilized egg to implant.

There are a few hormones involved in the ovarian and menstrual cycles: FSH (follicle-stimulating hormone), LH (luteinizing hormone), estrogen, and progesterone. Estrogen causes FSH and LH to spike right before ovulation, and the high levels of LH trigger ovulation. After ovulation, progesterone levels rise and inhibit the development of another egg (a mystery tool that will help us later!) to give the released egg time to travel through the fallopian tubes and get fertilized.

If semen is released into the vagina during intercourse, sperm cells can swim into the cervix towards the fallopian tubes. If it encounters an egg, the egg and sperm will join together in a process called fertilization. This is what we are trying to prevent. Sperm can stay active in the uterus and fallopian tubes for up to six days after it is released. Therefore, fertilization can occur up to six days after intercourse. If fertilization does not occur, the thick lining of the uterus is released during menstruation, and progesterone levels fall to allow for another ovulation.

A fertilized egg will move into the uterus while dividing into a cluster of cells called a blastocyst. The blastocyst will reach the uterus 3-4 days after fertilization and will stay in the uterus for 2-3 days. If the blastocyst attaches to the lining of the uterus, this is called implantation and signifies conception. The process of implantation takes 3-4 days. If a blastocyst does not implant in the uterus, it is released out of the vagina during menstruation, along with the thick lining of the uterus.

How the Pill Works

The Plan B pill is 1.5 milligrams of levonorgestrel, a synthetic form of progesterone (remember her?), and is currently sold over the counter at drugstores. The person at risk of an unwanted pregnancy takes one pill. Compared to other forms of birth control pills which are taken regularly, Plan B is a one-dose pill that is only taken once, after intercourse within 72 hours. The pill works using three main methods: delaying ovulation, preventing fertilization, and preventing implantation.

Plan B delays ovulation to prevent the mature egg from even encountering sperm. This happens by the synthetic progesterone inhibiting the spike of FSH and LH, and therefore preventing ovulation. This is the primary way that the pill is effective.

If the egg has already been released into the fallopian tubes, Plan B can still be effective by preventing fertilization. The pill prevents fertilization by slowing down the movement of both the egg and sperm by triggering the growth of cervical mucus, to reduce the probability that the two will meet to fertilize the egg. 

Finally, Plan B also claims to prevent implantation by thinning the lining of the uterus so that a fertilized egg will be unable to attach to the uterine lining. However, scientific studies have not proven this effect.

These three methods prevent conception from occurring, as opposed to an abortion pill (our Plan C), which would terminate a pregnancy that has already started by causing a miscarriage.

This is why Plan B needs to be taken within 72 hours, and as soon as possible for maximum effectiveness, because it will not terminate a pregnancy and will only target the factors that cause a pregnancy to prevent one from occurring in the first place.

Effectiveness of Plan B

The Plan B pill of 1.5 milligrams of levonorgestrel is only effective for those under 165 pounds. The options for those over 165 pounds include a copper IUD, which reduces the movement of sperm to prevent fertilization, and the ella morning after pill, which functions similarly to Plan B but contains 30 milligrams of ulipristal and is only available by prescription. IUDs that are implemented within 5 days of intercourse are 99% effective, whereas the ella pill is 98% effective.

Plan B is proven to be 87% effective at preventing pregnancy when taken within 72 hours of intercourse for those under 165 pounds. The sooner the pill is taken, the more effective it is, since the pill has the best chance of working when ovulation has not occurred yet. Effectiveness decreases if ovulation has occurred and decreases further if fertilization has occurred. Compared to other forms of contraception, Plan B is less effective. 

Cost and Accessibility

Other birth control pills need to be taken before intercourse regularly and other methods of birth control need to be implemented prior to intercourse, whereas Plan B is taken after intercourse if no birth control methods were used or other forms of contraception failed. Plan B can cost $50-$60 whereas generic levonorgestrel pills range from $15-$40. 

Plan B was previously only available by prescription, but was approved by the FDA to be over the counter for ages 18+ in 2006 and then expanded to all ages in 2013.

References

• planb.ca/en/how-plan-b-works/
• npr.org/sections/health-shots/2012/02/06/146358069/the-morning-after-pill-how-it-works-and-who-uses-it
• plannedparenthood.org/learn/pregnancy/how-pregnancy-happens#:~:text=The%20sperm%20and%20uterus%20work,an%20egg%2C%20it’s%20called%20fertilization.
• plannedparenthood.org/learn/birth-control/iud
• plannedparenthood.org/learn/morning-after-pill-emergency-contraception/whats-plan-b-morning-after-pill
• webmd.com/sex/birth-control/plan-b 
• britannica.com/science/levonorgestrel
• organismalbio.biosci.gatech.edu/growth-and-reproduction/animal-reproduction-ii-reproductive-structure-and-function/