Boys are more likely to be stillborn than girls
A large-scale study has found that boys are more likely to be stillborn than girls. The study reviewed more than 30 million births globally, and found the risk for stillbirth is about ten percent higher in boys. This equals a loss of about 100,000 male babies per year.
Published in the journal BMC Medicine, the results could help direct research into why some pregnancies go wrong. About a quarter of stillbirths have no known cause, of the remainder, many are linked to placental abnormalities. But, it is often unclear why abnormalities occur.
"The numbers speak for themselves - the disparity between male and female stillbirth rates is startling. Stillbirth is a common occurrence, even in rich countries with good healthcare systems: every day, eleven babies are stillborn in the UK. Uncovering why male babies are at higher risk could be a first step towards developing new approaches to prevention, including sex-specific management of high-risk pregnancies."
Fiona Mathews PhD, Department of Biosciences, College of Life and Environmental Sciences, Hatherly Laboratories, University of Exeter, United Kingdom
Although the incidence of stillbirth is lower in high-income countries, the study found that the percentage of increased risk of mortality in males was consistent across both high- and low-income countries.
The researchers recommend the routine recording of the sex of stillborn babies. Although the reasons for increased risk to male babies are not known, they could include developmental differences in the growth and function of the placenta, or increased sensitivity of male fetuses to environmental factors experienced by the mother, including obesity, smoking, advanced maternal age, and social deprivation.
The only exceptions to this global pattern were found in China and India — where sex-biased induced abortion is practiced. Here there were equal ratios of stillbirth in males and females and higher overall stillbirth risks than other countries. The mortality rates among females in these countries were 1.7 times greater than world wide.
Existing schemes to detect when babies are not growing properly have reduced stillbirth rates. They are based on recognizind early warning signs of possible trouble based on comparisons of the fetus's size and growth rate as compared to the mother's height, weight, ethnicity, and the number of previous live births. However, these comparisons rarely take into account the baby's gender. As male babies are, on average, larger than female, such information could help identify unusually small male babies who may be at high risk.
Stillbirth rates in high and low-income countries have declined very little in the past 15 years.
However, the United Kingdom has one of the highest stillbirth rates among wealthy nations, with one in 260 of all pregnancies resulting in stillbirth.
Stillbirth rates have changed little over the last decade, and a high proportion of cases are unexplained. This meta-analysis examined whether there are inequalities in stillbirth risks according to sex.
A systematic review of the literature was conducted, and data were obtained on more than 30 million birth outcomes reported in observational studies. The pooled relative risk of stillbirth was estimated using random-effects models.
The crude mean rate (stillbirths/1,000 total births) was 6.23 for males and 5.74 for females. The pooled relative risk was 1.10 (95% confidence interval (CI): 1.07–1.13). The attributable fraction in the whole population was 4.2% (95% CI: 3.70–4.63), and the attributable fraction among male fetuses was 7.8% (95% CI: 7.0–8.66). Study populations from countries with known sex-biased sex selection issues had anomalous stillbirth sex ratios and higher overall stillbirth risks than other countries, reflecting increased mortality among females.
Risk of stillbirth in males is elevated by about 10%. The population-attributable risk is comparable to smoking and equates to approximately 100,000 stillbirths per year globally. The pattern is consistent across countries of varying incomes. Given current difficulties in reducing stillbirth rates, work to understand the causes of excess male risk is warranted. We recommend that stillbirths are routinely recorded by sex. This will also assist in exposing prenatal sex selection as elevated or equal risks of stillbirth in females would be readily apparent and could therefore be used to trigger investigation.
The research was supported by a grant from The Wellcome Trust to study gender inequalities in early life health outcomes.
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