A recent meta-analysis finds reduced risk of preterm birth (PTB; <37 weeks gestational age) during the initial stage of COVID-19 in which infection rates remained relatively low but many societies imposed restrictions on movement. None of this work, however, examines sex-specific responses despite much literature on other ambient “shocks” which would predict male sensitivity. We use a conception cohort approach to explore potential sex-specific PTB responses in France, a country which imposed a lockdown in Spring 2020.

We applied interrupted time series methods using national data in France for 207 weeks among 1,403,284 males and 1,341,359 females conceived from 19 Jan 2016 to 6 Jan 2020.

For males in utero, the 1st COVID-19 societal lockdown corresponds with a -0.60 per 100 conception reduction in PTB cases per week, for 12 consecutive weeks (95% confidence interval [CI]: -.36, -.84). For females in utero, the PTB reduction is smaller (-0.40 reduction per 100 conceptions, for 10 consecutive weeks, 95% CI: -.15, -.61). A formal test of sex differences in the PTB response indicates a stronger reduction in male (vs. female) PTB during the lockdown (p=.001).

Explanations for the counterintuitive reduction in PTB during COVID-19 among cohorts in utero during Spring 2020 should consider mechanisms that disproportionately affect males.