Why, as a scientist, I signed an amicus brief for the US Supreme Court’s case on abortion.

The world is moving towards greater reproductive rights for women. More than 50 countries have liberalized their abortion laws in the past 25 years, informed by scientific research. Studies find that unsafe abortion is responsible for one in eight maternal deaths globally (E. Ahman and I. H. Shah Int. J. Gynaecol. Obstet. 115, 121–126; 2011), concentrated in low-income countries where abortion is illegal. Preventing unsafe abortion is a priority — 193 countries signed up to the United Nations Sustainable Development Goals, which call for reductions in maternal mortality.

Yet some countries, such as the United States, Poland and Nicaragua, are making access to abortion more difficult. Restrictions are passed on the basis of ideology or political motives, without considering scientific evidence about their impact. Science might not be able to decide philosophical questions about when life begins or when the rights of a fetus outweigh the agency of the person whose body is necessary for its growth and development. But it can tell us how access to abortion is affected by its legal status, and about the consequences when abortion is inaccessible. Science should weigh in on the often quoted yet seldom tested slogans of the abortion debate, because people’s well-being is at stake.

Consider this argument: ‘One cannot ban abortion; one can only ban safe abortion.’ This can be tested. When abortion is illegal, pregnant people are more likely to resort to unsafe methods. But some circumvent the law in ways that are safe. Those with the means travel to places where abortion is legal, and others take safe medications, approved by the World Health Organization, to terminate their pregnancies outside the formal health system. In Latin America, where self-managed abortion is widespread, large decreases in mortality from unsafe abortion have been documented without widespread changes to restrictive abortion laws (see go.nature.com/3d6gspd).

But there’s another consequence that should be investigated — when people are unable to get a legal abortion, they are more likely to carry unwanted pregnancies to term. It is estimated that 70% of unintended pregnancies end in abortion in places where it is legal, compared with about 50% where it is not (J. Bearak et al. Lancet Glob. Health 8, E1152–E1161; 2020). I am a demographer who gathers data and creates quantitative models to assess how unintended pregnancies affect the well-being of women, children and families. My work shows that there are serious ramifications.

Most of my evidence is drawn from the Turnaway Study, which I led. My team and I followed almost 1,000 women for five years after they sought an abortion in the United States, comparing the health and socio-economic consequences of receiving an abortion or being denied one. We found serious physical health consequences from continued pregnancy and childbirth, including death. Women and their existing and subsequent children also experienced greater economic and other hardship when abortion was denied. Women were more likely to continue to be exposed to intimate-partner violence, less likely to have an intended pregnancy under better circumstances later, and less likely to achieve their own aspirations...

... Studies in other countries where abortion is legal — Colombia, Tunisia, South Africa and Nepal — have found that many women are turned away because of difficulties including a lack of trained clinicians and low knowledge of the law. Some get an abortion outside the legal system, sometimes with serious medical complications. Others plan to carry the pregnancy to term, and anticipate hardships. A woman in Tunisia remarked that she did not have clothes for a newborn. “Four children, and a fifth one on top! Where are we heading this way? Poverty and tyranny” (S. Hajri et al. PLoS ONE 10, e0145338; 2015). An 18-year-old in Colombia who would not be able to continue her studies once she had a baby said: “I will no longer be able to be young” (T. DePiñeres et al. Reprod. Health 14, 133; 2017).

There is much more science to be done on abortion access. What is the impact of gestational limits? Who crosses borders to get care? What information, support and services help people to use abortion medications safely...