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The Science of Plan B

November 16, 2022

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/ 

 

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About Author

Aparna is a 3rd year computer science major, focused on media and modeling & simulation. In her free time, she likes to doodle, read fantasy & mystery novels, and waste time on TikTok.

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.