A test of oscillation in the human secondary sex ratio

Sex ratio at birth across 100 years in Sweden and risk of cardiovascular disease and all-cause mortality - a national register study

The human sex ratio at birth (SRB) undergoes temporary changes around a mean proportion of 0.51 male births. SRB has been well studied for historical, geographical, and secular trends, but until now not linked to health outcomes in the total population, e.g. for cardiovascular disease (CVD) or mortality during follow-up of birth cohorts. We used linkage analysis based on national registers in Sweden that cover all births from 1900 to 2016. SRB at birth was calculated by every 10-year birth cohort in all survivors living in 1997 for a follow-up analysis of risk of CVD and mortality with data from national registers. When the highest quartile of SRB was used as reference, a slightly increased risk of fatal CVD (HR 1.03 (95% confidence intervals, CI): 1.02-1.04), non-fatal CVD (HR 1.01; 95%CI: 1.01-1.02) and mortality (HR 1.02; 95%CI, 1.01-1.03) was found after full adjustments in men belonging to the lowest SRB quartile. A similar pattern was also found for fatal CHD in women. in the lowest SBR quartile compared to the highest, HR 1.03 (95%CI: 1.02-1.05). In conclusion, in birth cohorts with a relatively lower than expected number of males born, long-term adverse health effects were observed with slightly increased cardiovascular risk and total mortality at the population level. This could indicate that men belonging to so-called "culled cohorts" in a developed country during the 20th century are characterized by a slightly increased risk that could reflect negative early life influences and environmental exposures in pregnant women resulting in selective loss of male embryos or fetuses. In a public health perspective SRB could be of some importance to monitor as an aspect of birth statistics linked to relatively minor population health effects.

Effects of male hepatitis B virus infection and serostatus on sperm quality, pregnancy outcomes, and neonatal outcomes following intrauterine insemination

OBJECTIVE

To evaluate the impact of male hepatitis B virus (HBV) infection and serostatus on sperm quality, pregnancy outcomes, and neonatal outcomes following intrauterine insemination for infertility.

DESIGN AND METHODS

We retrospectively analyzed data from 962 infertile couples undergoing intrauterine insemination treatment at a single center. The case group comprised 212 infertile couples with male HBV infection, and the control group comprised 750 noninfected infertile couples. The couples were further divided into subgroups according to their hepatitis B e antigen (HBeAg)/anti-HBe status: hepatitis B surface antigen (HBsAg)+HBeAg- (group A), HBsAg+HBeAg+ (group B), and HBsAg-HBeAg- (control group). The main outcome parameters, including the seminal parameters, clinical pregnancy rate, miscarriage rate, live birth rate, preterm delivery rate, multiple pregnancy rate, delivery type, birth weight, and sex ratio, were compared.

RESULTS

A lower sperm acrosin activity, higher cesarean rate, and newborn sex ratio were observed in the HBV-infected group and group A in comparison with the control group (P < 0.05). However, the standard sperm parameters, clinical pregnancy rate, miscarriage rate, live birth rate, preterm delivery, and birth weight showed no statistically significant differences among the groups.

CONCLUSION

Male HBV infection does not adversely impact standard sperm parameters or pregnancy outcomes but can influence sperm acrosin activity and some neonatal outcomes. Moreover, the effect may vary among different HBV serostatuses.

The impact of catastrophic events on the sex ratio at birth: A systematic review

OBJECTIVE

The impact of maternal stress on birth outcomes is well established in the scientific research. The sex ratio at birth (SRB), namely the ratio of male to female live births, shows significant alteration when mothers experience acute stress conditions, as proposed by the Trivers-Willard Hypothesis. We aimed to synthetize the literature on the relationship between two exogenous and catastrophic stressful events (natural disasters and epidemics) and SRB.

METHODS

A systematic search was run in Scopus, PubMed, Web of Science, and Cochrane Library, until March 9, 2023. The search produced 1336 articles and 25 articles met the inclusion criteria. We found seven case-control studies and 18 observational studies. Most of studies investigated the impact of earthquakes and other natural disasters. Only seven studies examined the effect of epidemics or pandemics.

RESULTS

The results of the studies seem inconsistent, as 16 studies found a decline in SRB, three found a rise, four did not record any change and two studies gave contradictory results. The period and population analyzed, the source of information, the method of variance analysis in the SRB, and the failure to assess confounding variables may have influenced the incongruence of the results.

CONCLUSION

Our findings contribute to improve the knowledge about the relationship between socio-ecological factors and SRB. Future studies should investigate the mechanisms by which this relationship impacts public health, in particular the health of pregnant women and their newborn, through an accurate and consistent methodology that also includes confounding factors.

Severe cyclones and sex-specific birth outcomes in Queensland, Australia: An interrupted time-series analysis

OBJECTIVES

A male is less adaptable to biological stressors than a female fetus with consequent higher morbidity and mortality. Adverse birth outcomes increase and male livebirths decrease after environmental disasters, economic crises, and terrorist events. We hypothesized the ratio of male to female livebirths would decrease in areas affected by severe tropical cyclones (TCs) in Queensland, Australia. Additionally, in male livebirths, there would be an increase in preterm and low birthweight births. Lastly, we hypothesized that the pregnancy stage at which exposure occurred would modify the association between TC exposure and observed outcomes.

METHODS

Interrupted time series analysis was used to analyze Queensland administrative birth records from July 2007 to June 2018 for significant changes in the sex ratio at birth, measured as the proportion of male livebirths. Adjusted generalized linear models were fitted to births in areas affected by two category five TCs: cyclones Yasi (February 2011) and Marcia (February 2015). To explore male mortality and morbidity risk, additional analysis was conducted on the proportion of male stillborn, low birthweight, and preterm births. The association between estimated pregnancy stage during the TC and the proportion of male births was also analyzed.

RESULTS

Contrary to our hypothesis, increases in the proportion of male livebirths were observed following early-pregnancy exposure to cyclone Yasi and mid-pregnancy exposure to Marcia, although the latter was not statistically significant. No significant changes were observed in proportions of male stillborn, low birthweight, and preterm births.

CONCLUSIONS

This study found a significant association between severe TCs and sex ratio at birth. The stage of pregnancy at which maternal stressors were experienced modified this association. Among people exposed in early to mid-pregnancy, the proportion of male births was higher. This may be because of differential loss of females in utero. Studying sex differences in birth outcomes provides insight into in utero vulnerabilities associated with environmental stressors. Climate change is increasing the intensity and frequency of natural disasters. Understanding fetal vulnerability to environmental stressors will provide crucial information supporting early life health interventions that mitigate the immediate and long-term effects.

No Local Birth Sex Ratio Changes following the August 2019 Shootings in Montgomery County, Ohio, and in El Paso County, Texas

OBJECTIVE

In humans, males are born slightly in excess of females. Many factors have been shown to affect this ratio, including stressful events such as terrorist attacks. Two shootings occurred in early August 2019 in the Oregon District in Dayton, Montgomery County, Ohio, and in El Paso County, Texas, in the USA. This study was carried out in order to identify whether there were any effects on sex ratio at birth at the state or county level 3-5 months later.

SUBJECT AND METHODS

Births by sex, month of birth (2015-2019), and county were obtained for Ohio and Texas from the website of the Centers for Disease Control and Prevention. Ordinary linear logistic regression was used to assess the time trend in the probability of boys and to investigate changes in the trend functions. Poisson regression (SAS GENMOD) and linear logistic regression using SAS procedure LOGISTIC was applied.

RESULTS

This study analyzed 2,623,714 live births, 1,939,938 in Texas (sex odds [SO] 1.044) and 683,776 in Ohio (SO 1.045). The only significant effect noted was seasonality (month) at the state level.

CONCLUSION

It has been postulated that male fetal loss in pregnant women during stressful periods may occur in accordance with the Trivers-Willard hypothesis. Several studies have found significant effects after terrorist attacks in the USA (as well as in other countries), but this study did not reveal such effects. This may be due to several reasons including underpowered datasets and the possibility that populations may be becoming relatively immured to these events.

Twinning during the pandemic: Evidence of selection in utero

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.

Observable variations in human sex ratio at birth

The human sex ratio at birth (SRB), defined as the ratio between the number of newborn boys to the total number of newborns, is typically slightly greater than 1/2 (more boys than girls) and tends to vary across different geographical regions and time periods. In this large-scale study, we sought to validate previously-reported associations and test new hypotheses using statistical analysis of two very large datasets incorporating electronic medical records (EMRs). One of the datasets represents over half (∼ 150 million) of the US population for over 8 years (IBM Watson Health MarketScan insurance claims) while another covers the entire Swedish population (∼ 9 million) for over 30 years (the Swedish National Patient Register). After testing more than 100 hypotheses, we showed that neither dataset supported models in which the SRB changed seasonally or in response to variations in ambient temperature. However, increased levels of a diverse array of air and water pollutants, were associated with lower SRBs, including increased levels of industrial and agricultural activity, which served as proxies for water pollution. Moreover, some exogenous factors generally considered to be environmental toxins turned out to induce higher SRBs. Finally, we identified new factors with signals for either higher or lower SRBs. In all cases, the effect sizes were modest but highly statistically significant owing to the large sizes of the two datasets. We suggest that while it was unlikely that the associations have arisen from sex-specific selection mechanisms, they are still useful for the purpose of public health surveillance if they can be corroborated by empirical evidences.

The human sex ratio at birth (SRB), usually slightly greater than 1/2, have been reported to vary in response to a wide array of exogenous factors. In the literature, many such factors have been posited to be associated with higher or lower SRBs, but the studies conducted so far have focused on no more than a few factors at a time and used far smaller datasets, thus prone to generating spurious correlations. We performed a series of statistical tests on 2 large, country-wide health datasets representing the United States and Sweden to investigate associations between putative exogenous factors and the SRB, and were able to validate a set of previously-reported associations while also discovering new signals. We propose to interpret these results simply as public health indicators awaiting further empirical confirmation rather than as implicated in (adaptive) sexual selection mechanisms.

Maternal Diabetes and Infant Sex Ratio

PURPOSE OF REVIEW

Evolutionary hypotheses on the ratio of males to females at birth posit that women terminate pregnancies with low likelihood of surviving and producing grandchildren. Thus, females are preferred to males under unfavorable conditions. Much of this literature has focused on catastrophic disruptions that induce maternal stress and result in fewer males. Diabetes may similarly affect the sex ratio.

RECENT FINDINGS

A male bias at birth among infants born to women with GDM is widely recognized; mild hyperglycemia experienced early in pregnancy may signal favorable conditions and warrant investment in males. There are sparse data on women with pregestational diabetes, but some evidence for a female bias born to those with type 1 diabetes and severe hyperglycemia (i.e., requiring insulin). Disease-related maternal stress in these women may lead to the selective termination of male fetuses. Further examination of pregestational diabetes stands to contribute to scientific understanding of the sex ratio.

The Coupon Collection Behavior in Human Reproduction

There is evidence in humans for genetic influences on the probability (Pboy) that a birth yields a boy instead of a girl [1-6], suggesting a potential variation of Pboy among families. To quantify this variation, we analyze the survey data from over 300,000 UK Biobank participants primarily born between 1940 and 1970 [7]. Surprisingly, the proportion of male children in a family, or sex ratio (SR), has a significantly smaller among-family variation than expected under a uniform Pboy. We propose that this phenomenon results from reproductive behaviors reflecting a preference for having children of both sexes, much like the coupon collector's problem in probability theory where collecting a complete set of distinct coupons is considered a win. We find that the observed deficit in SR variation is explainable by 3.3% of "coupon-collecting" families. Consistently, significantly more families than expected have all children of the same sex except for the child born last. This trend is more pronounced in the late than the early half of the families in the data, suggesting an increasing popularity of this behavior. Analysis of a Dutch genealogical dataset spanning the past 4 centuries reveals higher-than-expected SR variations over much of the history; only after 1940 did the SR variation drop below the expectation. We conclude that a significant fraction of couples now exhibit the coupon collection behavior in reproduction such that SR is more homogeneous among families than expected by chance.