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The Science of Abortion 

Abortion is controversial but ubiquitous: 1 in 4 American women will have an abortion by the age of 45,1 and, around the world, 40-50 million pregnancies2 end in abortion every year.

Anti-abortion activists have long invoked moral and religious arguments to convince others that abortion is wrong, wrong, wrong, equating it to murder. And, for some pro-lifers, not just abortion, but anything that interferes with the development of a potential human life is considered murder, even masturbation. This led to the reductio ad absurdum of Monty Python’s song “Every Sperm Is Sacred.”

Some abortion activists seem not to have thought this through carefully. In a book I read long ago about an abortion clinic, one of the most vocal demonstrators picketing the clinic was a man adamantly against all abortions until he found out his own teenage daughter was pregnant. That was too close to home for comfort. It made him see things in a different light. He promptly changed his tune, arranging for his daughter to have an abortion rather than have her life ruined by an unwanted pregnancy. This reminded me of Animal Farm, where all animals are equal, but some are more equal than others. Did that father believe that all abortions are equally wrong but that they are somehow less equally wrong for his own progeny than for other women?

Someone came up with a clever thought experiment designed to help people clarify their thinking about potential human lives: A hospital is on fire. You have only a brief window of opportunity to get in and out safely. If you turn left, you can go to the nursery and rescue a newborn baby. If you turn right, you can go to the lab and rescue six embryos frozen in liquid nitrogen. If you really believe those potential lives are valuable, wouldn’t it make more sense to save six lives instead of one? What would you do? How would you feel about it? If you rescue the embryos, what could you possibly say to justify your choice to the mother whose newborn baby you have chosen to let die in the fire? Won’t she call you a murderer, or, at least, morally and legally culpable through inaction?

Since most women who request abortions don’t think of themselves as murderers, anti-abortion activists needed a more persuasive deterrent, so they have come up with a new tactic: claiming that abortions lead to serious adverse physical and mental health effects for the mothers. Never mind that science has extensively studied potential adverse effects and has proclaimed legal abortion safe. The findings of the studies were mostly negative, and the results of the few positive studies were questionable because of methodological flaws, such as unreliable self-reports, outdated surveying procedures, and failure to consider possible confounders. Perhaps the activists have some reason to believe the positive studies are accurate despite their obvious flaws. Motivated reasoning? Perhaps they think it is acceptable to lie. Maybe they believe the ends justify the means if lives are saved.

When Does the Fetus Become a Person and Have Human Rights? When Does Life Begin?

These questions remain mired in controversy. Many people hold that life begins at the moment of conception. But conception is not a moment, it is a process that unfolds over several days. After ejaculation, millions of sperm move up through the woman’s genital tract. The lucky one that wins the race meets an ovum in the Fallopian tube, where the two join to form a single-celled zygote, which then divides to become a multi-celled embryo. The pellucid zone, a membrane surrounding the egg, hardens after penetration by the first sperm, preventing penetration by other sperm. The sex of the fetus is determined by whether the sperm has an X or a Y chromosome. Science has recently learned that the egg can help determine which sperm succeeds3 by releasing chemicals into the follicular fluid that surrounds the egg. The sperm contributes centrioles that facilitate cell division. As the zygote divides, the DNA of egg and sperm are combined, and genes are exchanged to create a unique individual that inherits genes from both parents. After several days, the fertilized egg travels down into the uterus, burrowing into the uterine wall in the process known as implantation.

The Catholic Church holds that life begins with ensoulment, but “soul” is a religious concept, and, in any case, there is no way to determine whether a soul is present in an embryo or fetus. Aristotle believed that the soul entered a male embryo at 40 days and a female embryo at 90 days; this claim was not based on evidence and makes no sense. As abortion became legal in some jurisdictions, lawyers got into the act and muddied the waters. A convenient legal criterion asked whether the fetus could survive outside the body of the mother. This is not a workable solution, because as technology advances babies are enabled to survive at ever-earlier gestational ages and ever-lower birth weights. Another suggestion was that life begins with “quickening,” when the mother becomes aware of fetal movements; but this is too variable to be of any practical use. Some have argued that life begins when the baby takes its first breath. Theologians, philosophers, scientists, lawyers, and others have never been able to reach a consensus.

Legal vs. Illegal Abortions

Legal abortions are safe. Illegal abortions are not. 19 million women have unsafe abortions each year and 68,000 of them die, mostly in developing countries. Causes of death4 include:

  • incomplete abortion (failure to remove or expel all pregnancy tissue from the uterus);
  • hemorrhage (heavy or prolonged bleeding);
  • infection;
  • uterine perforation (caused when the uterus is pierced by a sharp object);
  • damage to the genital tract and internal organs as a consequence of inserting dangerous objects into the vagina or anus.

Legalizing abortion doesn’t significantly change the number of abortions, but it does increase their safety: only two deaths5 were reported among legal abortions in the U.S. in 2018.

Short-Term Complications

Most abortions today are medical, done with the drugs mifepristone and misoprostol, rather than surgical, with vacuum aspiration or dilation and curettage. A study comparing the risk of complications from medical versus surgical abortions6 found adverse events in 20% of medical abortions and 5.6% of surgical abortions. Adverse events included heavy or prolonged bleeding, infection, physical damage, and incomplete evacuation of the uterus requiring another procedure. The study showed no differences between medical and surgical abortions in the rate of infection, thromboembolic disease, psychiatric morbidity, or death.

Long-Term Adverse Effects

Anti-abortion activists tend to avoid talking about short-term complications and instead focus on creating fears about long-term consequences that have been largely rejected by scientific studies or questioned because of poor methodology.

In 2018, the National Academies published an extensive report7 from a committee tasked with reviewing all the published data about the safety and quality of abortion care in the U.S. as of 2018. It focused on four putative areas of potential harm:

  • future childbearing and pregnancy outcomes (e.g., secondary infertility, spontaneous abortion and stillbirth, preterm birth, low birthweight);
  • risk of breast cancer;
  • mental health disorders;
  • premature death.

After pointing out the limitations of the literature due to selective recall bias and other methodological flaws, the report summarizes what the research has shown about each area of potential harm.

What Science Says About Risks From Abortion

Abortion does not have adverse consequences for subsequent pregnancies. It does not cause secondary infertility. In fact, research found just the opposite. A large registry-based study in 2016 in Finland8 compared women who had had an abortion to women who had not. Those with a prior abortion were significantly less likely to be treated for infertility (1.95 versus 5.14 percent).

Abortion does not increase the risk of ectopic pregnancy. Ectopic pregnancy is known to be associated with upper genital tract infection, but serious infection after abortion is rare, especially now that antibiotic prophylaxis is standard practice. Several literature reviews have concluded that abortion is not associated with an increased risk of ectopic pregnancy, although admittedly all the published reviews were methodologically flawed.

Abortion is not associated with an increased risk of preterm birth in subsequent pregnancies. Several studies found no association with adverse outcomes in subsequent pregnancies, including a large 2013 Scottish study9 that the committee said had a number of strengths compared to other studies.

Abortion does not increase the risk of hypertension of pregnancy or eclampsia. In fact, a 2013 study9 found that women who had had an abortion had a lower risk of hypertensive disease and a lower risk of preeclampsia.

Numerous studies have shown no link between breast cancer and abortion. The odds appeared to be lower10 for women who had had a prior abortion.

The committee said, “As a result of the inability to control for the many ways in which women who have unwanted pregnancies differ from those who do not, no clear conclusions regarding the association between abortion and long-term mortality can be drawn from these studies.”

Likewise with mental health. Since the science is so clear about the physical safety of abortion, activists have focused on alleged mental health consequences such as depression, anxiety, post-traumatic stress, and suicide. But, according to the Guttmacher Policy Review,11 “Neither the weight of the scientific evidence to date nor the observable reality of 33 years of legal abortion in the United States comports with the idea that having an abortion is any more dangerous to a woman’s long-term mental health than delivering and parenting a child that she did not intend to have or placing a baby for adoption.” Further, the “postabortion traumatic stress syndrome” that activists say is widespread is not recognized by either the American Psychological Association (APA) or the American Psychiatric Association.

In 1987, President Reagan directed U.S. Surgeon General C. Everett Koop (known for his strong opposition to abortion when he was appointed in 1981 to be the nation’s top public health doctor by a president strongly supported by the Religious Right) to examine the studies on the health effects of abortion and prepare a report. Fifteen months later, Koop wrote a letter advising the President that he would not be issuing a report because “the scientific studies do not provide conclusive data about the health effects of abortion on women.” At a congressional hearing, Koop said it was clear to him that the psychological effects of abortion are “minuscule” from a public health perspective. In 1989, an American Psychological Association review determined that legal abortion of an unwanted pregnancy “does not pose a psychological hazard for most women.”

What They Don’t Want You to Know

Anti-abortion activists are happy to frighten women with the alleged risks of abortion, but they are careful not to divulge this crucial information: whatever the risks of getting an abortion, it’s far riskier not to get an abortion. Pregnancy is known to be hazardous to health, and the risks of continued pregnancy and childbirth are well documented.

According to an article in Obstetrics and Gynecology,12 “Legal induced abortion is markedly safer than childbirth. The risk of death associated with childbirth is approximately 14 times higher than that with abortion. Similarly, the overall morbidity associated with childbirth exceeds that with abortion.”

Those are evidence-based facts, not political or religious dogma. END

  6. Niinimäki, M., Pouta, A., Bloigu, A., Gissler, M., Hemminki, E., Suhonen, S., & Heikinheimo, O. (2009). Immediate complications after medical compared with surgical termination of pregnancy. Obstetrics & Gynecology, 114(4), 795–804.
  7. National Academies of Sciences, Engineering, and Medicine. (2018). The Safety and Quality of Abortion Care in the United States. Washington, DC: The National Academies Press.
  8. Holmlund, S., Kauko, T., Matomäki, J., Tuominen, M., Mäkinen, J., & Rautava, P. (2016). Induced abortion – impact on a subsequent pregnancy in first-time mothers: A registry-based study. BMC Pregnancy and Childbirth, 16(1).
  9. Woolner, A., Bhattacharya, S., & Bhattacharya, S. (2013). The effect of method and gestational age at termination of pregnancy on future obstetric and Perinatal Outcomes: A register-based Cohort Study in Aberdeen, Scotland. BJOG: An International Journal of Obstetrics & Gynaecology, 121(3), 309–318.
  10. Goldacre, M. J. (2001). Abortion and breast cancer: A case-control record linkage study. Journal of Epidemiology & Community Health, 55(5), 336–337.
  12. Raymond, E. G., & Grimes, D. A. (2012). The comparative safety of legal induced abortion and childbirth in the United States. Obstetrics & Gynecology, 119(2, Part 1), 215–219.
About the Author

Harriet Hall, MD, the SkepDoc, is a retired family physician, former flight surgeon, and retired Air Force Colonel who writes about medicine, pseudoscience, alternative medicine, quackery, and critical thinking. She is a contributing editor and regular columnist for both Skeptic and Skeptical Inquirer magazines as well as a columnist for the Committee for Skeptical Inquiry online and an editor at, where she has written an article every Tuesday since its inception in 2008. She wrote the book Women Aren’t Supposed to Fly: The Memoirs of a Female Flight Surgeon. The full texts of all her many hundreds of articles can be read on her website

This article was published on August 23, 2022.

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