Maternal stress and sex ratio at birth in Sweden over two and a half centuries: a retest of the Trivers-Willard hypothesis

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.

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.

Income, stress, and sex ratios over 1895-2010

The medical literature has shown that populations under high stress have a lower sex ratio at birth (i.e. number of males for every 100 females). In this paper, I examine the relationship between income, as a source of economic stress, and the sex ratio at a subnational level for the 1895-2010 period. For this, I use census microdata from Argentina -a developing country that experienced rapid growth at the end of the 19th century and stagnated in recent decades- and I estimate from a two-way fixed effects model that exploits the wide temporal and geographic variability in income. The results show that as per capita income increases, the sex ratio at birth also increases. In particular, for every US$ 1,000 increase in per capita income, the sex ratio increases between 0.3 and 0.6 points. These findings make it possible to quantify the lost boys (i.e. those boys who were not born due to high economic stress on their parents) and constitute a call for attention in favor of the implementation of prenatal care policies -especially in periods of stagnation or income decline- to maintain a more balanced sex ratio.

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.

Girls start life on an uneven playing field

Background and objectives

Evolutionary research on the sex ratio at birth (SRB) has focused on explaining variability within and between populations, and whether parental fitness is maximized by producing daughters or sons. We tested predictors of SRB in a low-income setting, to understand whether girls differ from boys in their likelihood of being born into families with the capacity to invest in them, which has implications for their future health and fitness.

Methodology

We used data from a cluster randomized control trial from lowland rural Nepal (16 115 mother-child dyads). We applied principal component analysis to extract two composite indices reflecting maternal socio-economic and reproductive (parity, age) capital. We fitted mixed-effects logistic regression models to estimate odds ratios of having a girl in association with these individual factors and indices.

Results

The SRB was 112. Compared to the global reference SRB (105), there were seven missing girls per 100 boys. Uneducated, early-marrying, poorer and shorter mothers were more likely to give birth to girls. Analysing composite maternal indices, lower socio-economic and reproductive capital were independently associated with a greater likelihood of having a girl.

Conclusions and implications

In this population, girls start life facing composite disadvantages, being more likely than boys to be born to mothers with lower socio-economic status and reproductive capital. Both physiological and behavioural mechanisms may contribute to these epidemiological associations. Differential early exposure by sex to maternal factors may underpin intergenerational cycles of gender inequality, mediated by developmental trajectory, education and socio-economic status.

Sex ratio imbalance following blastocyst transfer is associated with ICSI but not with IVF: an analysis of 14,892 single embryo transfer cycles

PURPOSE

Assisted reproductive technology (ART) has an impact on secondary sex ratio (SSR), which is seemed to be elevated after blastocyst transfer (BT) but decreased following ICSI procedure. We aim to assess whether the higher SSR associated with BT could be influenced by fertilization method used.

METHODS

All consecutive IVF/ICSI cycles (fresh and frozen) involving single embryo transfer (SET) resulting in a live birth between 2015 and 2019 were retrospective analyzed. Logistic regression was used to model the effect on the SSR of maternal and specific ART characteristics.

RESULTS

Six thousand nine hundred twenty-two women were included with the crude SSR of 54.8%. The impact of BT on SSR is influenced by the fertilization method used. After adjustment for potential confounders, the SSR in the ICSI BT group was significantly higher when compared to ICSI cleavage-stage embryo SET (aOR 1.24; 95% CI 1.10-1.40, P < 0.001). However, this effect was not detected among SBT with IVF treatment (aOR 1.04; 95% CI 0.97-1.12, P = 0.260). Assessing blastocyst morphological parameters, high trophectoderm quality was significantly associated with elevated SSR (aOR 1.76, 95% CI 1.34-2.31 [A vs. C], and aOR 1.28, 95% CI 1.14-1.44 [B vs. C]). No significant difference was shown in expansion, inner cell mass, or days of blastocyst formation between male and female blastocysts.

CONCLUSIONS

The impact of BT on SSR could be influenced by the fertilization method used. The higher SSR was observed after BT with ICSI procedures but not with IVF. Interpretation of the findings is limited by the potential for selection and confounding bias.