Twinning during the pandemic

Abstract

Background and Objectives The suspicion that a population stressor as profound as the COVID-19 pandemic would increase preterm birth among cohorts in gestation at its outset has not been supported by data collected in 2020. An evolutionary perspective on this circumstance suggests that natural selection in utero, induced by the onset of the pandemic, caused pregnancies that would otherwise have produced a preterm birth to end early in gestation as spontaneous abortions. We test this possibility using the odds of a live-born twin among male births in Norway as an indicator of the depth of selection in birth cohorts. Methodology We apply Box-Jenkins methods to 50 pre-pandemic months to estimate counterfactuals for the 9 birth cohorts in gestation in March 2020 when the first deaths attributable to SARS-CoV-2 infection occurred in Norway. We use Alwan and Roberts outlier detection methods to discover any sequence of outlying values in the odds of a live-born twin among male births in exposed birth cohorts. Results We find a downward level shift of 27% in the monthly odds of a twin among male births beginning in May and persisting through the remainder of 2020. Conclusions and Implications: Consistent with evolutionary theory and selection in utero, birth cohorts exposed in utero to the onset of the COVID-19 pandemic yielded fewer male twins than expected. Lay Summary Our finding of fewer than expected male twin births during the onset of the COVID-19 pandemic provides more evidence that evolution continues to affect the characteristics and health of contemporary populations.

Selection in utero against male twins in the United States early in the COVID‐19 pandemic

“…The magnitude of our results (in terms of percentage reduction of male twins during COVID‐19 shelter‐in‐place policies) appears smaller than that reported for Norway, where odds of male twin births decreased by 27% (Catalano et al, 2021). We, however, caution against direct comparisons for two reasons.…”

“…Reports of selection in utero against male twins during stressful times appear in the epidemiologic literature (Catalano et al, 2012; Karasek et al, 2015). Research from Norway suggests that the onset of the COVID‐19 pandemic may have triggered selection in utero (Catalano et al, 2021). Using time‐series analyses, researchers detected fewer than expected male twins 2 months after the enactment of “shelter‐in‐place” policies.…”

“…First, the United States recorded an average of 5180 male twins and 155 688 male births per month over the last 5 years, which minimizes the risk that variability due to a low volume of male twin births induces spurious findings. Second, surveillance epidemiology in the United States detected fewer preterm births than expected from history among the 6 monthly cohorts conceived in the United States from July through December 2019 (Catalano et al, 2021; Gemmill et al, 2022; Harvey et al, 2021) and in gestation during the “shelter‐in‐place” phase of the COVID‐19 pandemic. This circumstance raises the possibility that if selection in utero contributed to “missing” preterm births during COVID‐19, then at least 1 of those conception cohorts should also exhibit fewer than expected male twin births.…”