The relevance of the epidemiology of human sex ratios at birth to some medical problems

Maternal serum cortisol levels during pregnancy differ by fetal sex

BACKGROUND

Males and females have different patterns of fetal growth, resulting in different sizes at birth. Increased maternal cortisol levels in pregnancy negatively impact fetal growth. However, it is unknown whether sexual dimorphism displays differences in maternal cortisol levels already during early pregnancy and to what extent it explains sex differences in intra-uterine growth. The present cross-sectional study investigated whether fetal sex was associated with the level of maternal serum total cortisol in first half of pregnancy and its contribution to sex differences in fetal growth.

METHOD

The study population comprised 3049 pregnant women from the Amsterdam Born Children and their Development (ABCD)-cohort). Total serum cortisol levels were determined during pregnancy. Multivariable linear regression was used to determine fetal sex differences in maternal cortisol levels and its association with sex differences in fetal growth measured as birth weight standardized for gestational age, parity and sex.

RESULTS

Maternal serum total cortisol increased during pregnancy from on average 390 ± 22 nmol/L (at 5th week) to 589 ± 15 nmol/L (at 20th week). Women carrying a female fetus had higher maternal total cortisol levels. This sex difference was not significant before the 11th week; at the 12th week the difference was 15 ± 7 nmol/L which increased to 45 ± 22 nmol/L at the 20th week (p-for-interaction=0.05). Maternal total cortisol levels were associated with birth weight (ß:-0.22;P < 0.001). However, sex differences in birth weight were not explained by related maternal total cortisol levels.

CONCLUSION

The sexual dimorphic maternal serum total cortisol levels are apparent after the first trimester but do not explain the different patterns of fetal growth.

Sex ratio at birth is associated with type 1 diabetes characteristics

AIMS

To assess the association between maternal diabetes characteristics and sex ratio at birth (SRB) in women with type 1 diabetes mellitus.

METHODS

We performed a case-control study. The study subjects were infants born alive to women with type 1 diabetes and singleton pregnancies. Cases and controls were defined as male and female newborns, respectively. SRB was analysed according to diabetes-related characteristics adjusting in a logistic regression analysis for maternal characteristics known to affect SRB in the general population.

RESULTS

The observed SRB (238 males/468 live births = 0.509) did not differ from the expected. In the logistic regression analysis, SRB was significantly associated with three diabetes characteristics: (1) diabetes duration, with odds ratios (ORs) for a live male newborn = 1.22 (95 % confidence interval (CI), 0.66-2.24 for ≤5 years, OR 2.79 (95 % CI 1.36-5.74) for >20 years; (2) mean first-trimester glycated haemoglobin, with OR 1.98 (95 % CI 1.09-3.62) for ≤6.7 % (50 mmol/mol) and OR 2.61 (95 % CI 1.16-5.85) for >8.2 % (66 mmol/mol) and (3) mean first-trimester insulin dose, with OR 0.70 (95 % CI 0.36-1.38) for ≤0.5 IU/kg/day and OR 0.18 (95 % CI 0.05-0.59) for >1.0 IU/kg/day.

CONCLUSIONS

We conclude that SRB in this cohort is independently associated with three diabetes characteristics. These associations are to be confirmed.

PARTIALLY CONSTRAINED SEX ALLOCATION AND THE INDIRECT EFFECTS OF ASSISTED REPRODUCTIVE TECHNOLOGIES ON THE HUMAN SEX RATIO

Infertility affects around 15% of human couples and in many countries approximately 1-4% of babies are born following Assisted Reproductive Technologies (ART). Several ART techniques are used and these differentially affect the sex ratio of offspring successfully produced. These direct effects on sex ratio also have the potential to influence, indirectly, the sex ratios of offspring born to untreated couples. This is of concern because human sex ratio bias may adversely affect public health. Here the extent of indirect effects of ART that could operate, via Fisherian frequency-dependent natural selection, on the progeny sex ratio of unassisted members of a population is heuristically modelled. Given the degrees to which ART techniques bias sex ratios directly, it is predicted that well over 20% of couples would have to reproduce via ART for there to be any discernible effect on the sex ratios produced, in response, by the remainder of the population. This value is greater than the estimated prevalence of infertility problems among human couples. It is concluded that providing ART to couples with fertility problems does not currently generate significant ethical issues or public health concern in terms of indirect effects on the offspring sex ratios of untreated couples.

Do differences in Toxoplasma prevalence influence global variation in secondary sex ratio? Preliminary ecological regression study

Sex of the fetus is genetically determined such that an equal number of sons and daughters are born in large populations. However, the ratio of female to male births across human populations varies significantly. Many factors have been implicated in this. The theory that natural selection should favour female offspring under suboptimal environmental conditions implies that pathogens may affect secondary sex ratio (ratio of male to female births). Using regression models containing 13 potential confounding factors, we have found that variation of the secondary sex ratio can be predicted by seroprevalence of Toxoplasma across 94 populations distributed across African, American, Asian and European continents. Toxoplasma seroprevalence was the third strongest predictor of secondary sex ratio, β = −0·097, P < 0·01, after son preference, β = 0·261, P < 0·05, and fertility, β = −0·145, P < 0·001. Our preliminary results suggest that Toxoplasma gondii infection could be one of the most important environmental factors influencing the global variation of offspring sex ratio in humans. The effect of latent toxoplasmosis on public health could be much more serious than it is usually supposed to be.

Studies of human sex ratios at birth may lead to the understanding of several forms of pathology

This article deals with the problem of the causes of the variation of sex ratio (proportion male) at birth. This problem is common to a number of areas in biology and medicine, for example, obstetrics, neurology/psychiatry, parasitology, virology, oncology, and teratology. It is established that there are signifi cantly biased, but unexplained, sex ratios in each of these fields. Yet workers in them (with the possible exception of virology) have regarded the problem as a minor loose end, irrelevant to the field's major problems. However, as far as I know, no one has previously noted that unexplained biased sex ratios occur, and thus pose (perhaps similar) problems, in all these fields. Here it is suggested that similar sorts of solutions apply in each. Further research is proposed for testing each solution. If the argument here is substantially correct across this range of topics, it may lead to an improved understanding not only of sex ratio but also of some of the pathologies in these specialties.

Sex ratios among infants with birth defects, National Birth Defects Prevention Study, 1997-2009

A small number of population-based studies have examined sex differences among infants with birth defects. This study presents estimates of sex ratio for both isolated cases and those with multiple congenital anomalies, as well as by race/ethnicity. Male-female sex ratios and their 95% confidence intervals were calculated for 25,952 clinically reviewed case infants included in the National Birth Defects Prevention Study (1997-2009), a large population-based case-control study of birth defects. The highest elevations in sex ratios (i.e., male preponderance) among isolated non-cardiac defects were for craniosynostosis (2.12), cleft lip with cleft palate (2.01), and cleft lip without cleft palate (1.78); the lowest sex ratios (female preponderance) were for choanal atresia (0.45), cloacal exstrophy (0.46), and holoprosencephaly (0.64). Among isolated cardiac defects, the highest sex ratios were for aortic stenosis (2.88), coarctation of the aorta (2.51), and d-transposition of the great arteries (2.34); the lowest were multiple ventricular septal defects (0.52), truncus arteriosus (0.63), and heterotaxia with congenital heart defect (0.64). Differences were observed by race/ethnicity for some but not for most types of birth defects. The sex differences we observed for specific defects, between those with isolated versus multiple defects, as well as by race/ethnicity, demonstrate patterns that may suggest etiology and improve classification.

Economic stress or random variation? Revisiting German reunification as a natural experiment to investigate the effect of economic contraction on sex ratios at birth

BACKGROUND

The economic stress hypothesis (ESH) predicts decreases in the sex ratio at birth (SRB) following economic decline. However, as many factors influence the SRB, this hypothesis is difficult to test empirically. Thus, researchers make use of quasi-experiments such as German reunification: The economy in East, but not in West Germany, underwent a rapid decline in 1991. A co-occurrence of a decline in the East German SRB in 1991 has been interpreted by some as support for the ESH. However, another explanation might be that the low SRB in 1991 stems from increased random variation in the East German SRB due to a drastically reduced number of births during the crisis. We look into this alternative random variation hypothesis (RVH) by re-examining the German case with more detailed data.

METHODS

Our analysis has two parts. First, using aggregate-level birth register data for all births in the period between 1946 and 2011, we plot the quantum and variance of the SRB and the number of births and unemployment rates, separately for East and West Germany, and conduct a time series analysis on the East German SRB over time. Second, we model the odds for a male birth at the individual level in a multiple logistic regression (1991-2010, ~13.9 million births). Explanatory variables are related to the level of the individual birth, the mother of the child born, and the regional economic context.

RESULTS

The aggregate-level analysis reveals a higher degree of variation of the SRB in East Germany. Deviations from the time trend occur in several years, seemingly unrelated to economic development, and the deviation in 1991 is not statistically significant. The individual-level analysis confirms that the 1991-drop in the East German SRB cannot directly be attributed to economic development and that there is no statistically significant effect of economic development on sex determination in East or West Germany.

CONCLUSION

Outcomes support the RVH but not the ESH. Furthermore, our results speak against a statistically significant effect of the reunification event itself on the East German SRB. We discuss the relative importance of behavioral and physiological responses to macro-level stressors, a distinction that may help integrate previously mixed findings.

Effects of assisted reproductive technologies on human sex ratio at birth

OBJECTIVE

To investigate the effect of assisted reproductive technology (ART) treatments on the sex ratio of babies born.

DESIGN

Assessment of direct effects of assisted conception through retrospective data analysis on the progeny sex ratio of treated women in the United Kingdom.

SETTING

The study uses the anonymized register of the Human Fertilisation and Embryology Authority.

PATIENT(S)

A total of 106,066 babies of known gender born to 76,994 treated mothers and 85,511 treatment cycles between 2000 and 2010 in the United Kingdom.

INTERVENTION(S)

Intrauterine insemination, IVF, or intracytoplasmic sperm injection (ICSI).

MAIN OUTCOME MEASURE(S)

Sex ratio of babies born.

RESULT(S)

Intrauterine insemination, IVF, and ICSI lead to different sex ratios, highest after IVF (proportion male = mean 0.521 ± confidence interval 0.0056) and lowest under ICSI embryo transfer (0.493 ± 0.0031). In addition, for both ICSI and IVF, transferring embryos at a later stage (blastocyst) results in approximately 6% more males than after early cleavage-stage ET.

CONCLUSION(S)

Because the cumulative number of IVF babies born is increasing significantly in Britain and elsewhere, more research is needed into the causes of gender bias after ART and into the public health impact of such gender bias of offspring born observed on the rest of the population.