Hans Rosling (1948–2017), physician and epidemiologist, famously upturned assumptions widely held by the public and by the development community — assumptions that, thanks to US President Donald Trump, are back in the spotlight. A recurring theme of Rosling's was that family sizes have been shrinking even though child survival rates have improved (see, for example, Nature 540, 330–333; 2016). Although reducing infant mortality is an unfettered good, we are concerned that key family-planning initiatives could be lost if governments uncritically assume that it leads to people having fewer children.

Rosling's global statistics on total fertility rate and infant mortality rate do not indicate causality, neither are they necessarily correlated. Although such a correlation holds for Asia, it does not in Africa. The infant mortality rate in Niger, for example, has fallen by two-thirds since the 1980s but the country's total fertility rate has risen slightly, leading to a predicted population explosion from 20 million in 2015 to 72 million by 2050 (see go.nature.com/2j7rzcv).

In our view, President Trump's reinstatement of the Mexico City Policy (the 'global gag rule', which denies US funds to foreign non-governmental organizations associated with abortion counselling and referrals) will be disastrous for population control and for women's health. When President George W. Bush imposed a milder version of this rule in 2001, the unofficial abortion rate in African counties rose steeply (E. Bendavid et al. Bull. World Health Organ. 89, 873–880; 2011). US politicians must grasp that the best way to prevent abortions is to invest heavily in accessible family planning.