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Background and objectives Selection in utero predicts that population stressors raise the standard for how quickly fetuses must grow to avoid spontaneous abortion. Tests of this prediction must use indirect indicators of fetal loss in birth cohorts because vital statistics systems typically register fetal deaths at the 20th week of gestation or later, well after most have occurred. We argue that tests of selection in utero would make greater progress if researchers adopted an indicator of selection against slow-growing fetuses that followed from theory, allowed sex-specific tests and used readily available data. We propose such an indicator and assess its validity as a dependent variable by comparing its values among monthly birth cohorts before, and during, the first 10 months of the COVID-19 pandemic in Sweden. Methodology We apply Box–Jenkins methods to 50 pre-pandemic birth cohorts (i.e. December 2016 through January 2020) and use the resulting transfer functions to predict counterfactual values in our suggested indicator for selection for ten subsequent birth cohorts beginning in February 2020. We then plot all 60 residual values as well as their 95% detection interval. If birth cohorts in gestation at the onset of the pandemic lost more slow-growing fetuses than expected from history, more than one of the last 10 (i.e. pandemic-exposed) residuals would fall below the detection interval. Results Four of the last 10 residuals of our indicator for males and for females fell below the 95% detection interval. Conclusions and implications Consistent with selection in utero , Swedish birth cohorts in gestation at the outset of the COVID-19 pandemic included fewer than expected infants who grew slowly in utero. [ABSTRACT FROM AUTHOR]

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 nine 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. [ABSTRACT FROM AUTHOR]