Twinning during the pandemic: Evidence of selection in utero

Preterm births among male and female conception cohorts in France during initial COVID-19 societal restrictions

PURPOSE

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.

METHODS

We applied interrupted time series methods using national data in France for 207 weeks among 1403,284 males and 1341,359 females conceived from 19 Jan 2016 to 6 Jan 2020.

RESULTS

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).

CONCLUSIONS

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

A novel indicator of selection in utero

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.

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

OBJECTIVES

We aim to contribute to the literature reporting tests of selection in utero. The theory of reproductive suppression predicts that natural selection would conserve mechanisms, referred to collectively as selection in utero, that spontaneously abort fetuses unlikely to thrive as infants in the prevailing environment. Tests of this prediction include reports that women give birth to fewer than expected male twins, historically among the frailest of infants, during stressful times. The onset of the COVID-19 pandemic in the United States in Spring 2020 demonstrably stressed the population. We test the hypothesis that conception cohorts in gestation at the onset of the pandemic in the United States yielded fewer than expected live male twin births.

METHODS

We retrieved deidentified data on the universe of live births in the United States from the National Center for Health Statistics birth certificate records. We applied Box-Jenkins time-series methods to the twin secondary sex ratio computed for 77 monthly conception cohorts spanning August 2013 to December 2019 to detect outlying cohorts in gestation at the onset of the pandemic.

RESULTS

The twin secondary sex ratio fell below expected values in three conception cohorts (i.e., July, September, and October 2019, all p < .05) exposed in utero to the onset of the pandemic.

CONCLUSIONS

Our results add to prior findings consistent with selection in utero. The role of selection in utero in shaping the characteristics of live births cohorts, especially during the COVID-19 pandemic, warrants further scrutiny.

COVID-19 induced birth sex ratio changes in England and Wales

Background

The sex ratio at birth (male live births divided by total live births) may be a sentinel health indicator. Stressful events reduce this ratio 3-5 months later by increasing male fetal loss. This ratio can also change 9 months after major population events that are linked to an increase or decrease in the frequency of sexual intercourse at the population level, with the ratio either rising or falling respectively after the event. We postulated that the COVID-19 pandemic may have affected the ratio in England and Wales.

Methods

Publicly available, monthly live birth data for England and Wales was obtained from the Office for National Statistics up to December 2020. Using time series analysis, the sex ratio at birth for 2020 (global COVID-19 onset) was predicted using data from 2012-2019. Observed and predicted values were compared.

Results

From 2012-2020 there were 3,133,915 male and 2,974,115 female live births (ratio 0.5131). Three months after COVID-19 was declared pandemic (March 2020), there was a significant fall in the sex ratio at birth to 0.5100 in June 2020 which was below the 95% prediction interval of 0.5102-0.5179. Nine months after the pandemic declaration, (December 2020), there was a significant rise to 0.5171 (95% prediction interval 0.5085-0.5162). However, December 2020 had the lowest number of live births of any month from 2012-2020.

Conclusions

Given that June 2020 falls within the crucial window when population stressors are known to affect the sex ratio at birth, these findings imply that the start of the COVID-19 pandemic caused population stress with notable effects on those who were already pregnant by causing a disproportionate loss of male fetuses. The finding of a higher sex ratio at birth in December 2020, i.e., 9 months after COVID-19 was declared a pandemic, could have resulted from the lockdown restrictions that initially spurred more sexual activity in a subset of the population in March 2020.

Exposure to the early COVID-19 pandemic and early, moderate and overall preterm births in the United States: A conception cohort approach

BACKGROUND

The United States (US) data suggest fewer-than-expected preterm births in 2020, but no study has examined the impact of exposure to the early COVID-19 pandemic at different points in gestation on preterm birth.

OBJECTIVE

Our objective was to determine-among cohorts exposed to the early COVID-19 pandemic-whether observed counts of overall, early and moderately preterm birth fell outside the expected range.

METHODS

We used de-identified, cross-sectional, national birth certificate data from 2014 to 2020. We used month and year of birth and gestational age to estimate month of conception for birth. We calculated the count of overall (<37 weeks gestation), early (<33 weeks gestation) and moderately (33 to <37 weeks gestation) preterm birth by month of conception. We employed time series methods to estimate expected counts of preterm birth for exposed conception cohorts and identified cohorts for whom the observed counts of preterm birth fell outside the 95% detection interval of the expected value.

RESULTS

Among the 23,731,146 births in our study, the mean prevalence of preterm birth among monthly conception cohorts was 9.7 per 100 live births. Gestations conceived in July, August or December of 2019-that is exposed to the early COVID-19 pandemic in the first or third trimester-yielded approximately 3245 fewer moderately preterm and 3627 fewer overall preterm births than the expected values for moderate and overall preterm. Gestations conceived in August and October of 2019-that is exposed to the early COVID-19 pandemic in the late second to third trimester-produced approximately 498 fewer early preterm births than the expected count for early preterm.

CONCLUSIONS

Exposure to the early COVID-19 pandemic may have promoted longer gestation among close-to-term pregnancies, reduced risk of later preterm delivery among gestations exposed in the first trimester or induced selective loss of gestations.

Pandemic influenza and gender imbalance: Mortality selection before births

This study uses data from the 1918-1920 influenza pandemic in Japan along with newly digitized and complete census records on births and infant deaths to analyze mortality selection in utero. I find that fetal exposure to the influenza pandemic during the first trimester of the pregnancy decreases the proportion of males at birth. The results from mechanism analysis suggest that this decline in male births is associated with the deterioration of fetal and infant health. This result supports a wide range of existing literature on the long-run adverse effects of pandemic influenza.

A preliminary analysis of the secondary sex ratio decline after the COVID-19 pandemic in Japan

Objectives

The secondary sex ratio (SSR, i.e., the number of male births per 100 female births) has long been proposed as a sentinel health indicator. Studies have suggested that exogenous environmental stressors reduce SSR after 3 to 6 months (via disproportionate male fetus deaths) and after 9 months (via reduced male conception). We aimed to examine whether SSR declined after the coronavirus disease 2019 (COVID‐19) pandemic in Japan.

Methods

We used monthly vital statistics records collected between January 2013 and April 2021 (the ‐number of male and female live births, in particular). Using information reported before the pandemic struck Japan (i.e., January 2013 to January 2020), we predicted SSRs for the months after the pandemic (i.e., February 2020 to April 2021) and compared reported and predicted SSRs. We also stratified the analysis by including two groups of prefectures with different degrees of possible influence from the pandemic.

Results

We observed a significant reduction in SSR during December 2020 of 102.81 (i.e., 9–10 months after the pandemic affected the country), which was below the lower bound of the 95% prediction intervals (103.12–106.33). This was the lowest SSR observed during the 100‐month study period. In addition, the reduction in SSR during December 2020 was more pronounced in the more seriously affected prefectures.

Conclusion

In Japan, we found a significant reduction in SSR (i.e., fewer male live births) 9–10 months after COVID‐19 was declared a pandemic. This suggests the onset of the COVID‐19 pandemic was a significant population‐level stressor.