The pandemic preterm paradox: a test of competing explanations.

Epidemiologists have long argued that side effects of the stress response include preterm birth. Research reports that fear of lethal infection stressed pregnant persons at the outset of the coronavirus disease (COVID-19) pandemic and that "shutdowns" and "social distancing" impeded access to social support and prenatal care. The decline in preterm births in high-income countries, including the United States, during the early months of the pandemic therefore poses a paradox for science. Explanations of this "pandemic preterm paradox" remain untested. We applied time-series modeling to data describing 80 monthly conception cohorts begun in the United States from July 2013 through February 2020 to determine which of 3 explanations most parsimoniously explained the paradox. We infer that "prior loss," or the argument that an increase in spontaneous abortions and stillbirths depleted the population of fetuses at risk of preterm birth, best explains data currently available. We describe the implications of these results for public health practice.