The sex ratio at conception is even, but slightly more boys are born than girls worldwide because more females die in the womb, according to a new study by researchers in Britain and the United States.
It has long been known that the sexes aren't born in equal numbers. In 2013 in Germany, for example, 349,820 boys and 332,249 girls were born, according to the national Statistics Office - a ratio of 51 to 49.
Although many experts have long assumed that more boys are conceived than girls, "the sex ratio at conception in humans is unknown despite hundreds of years of speculation and research," write the researchers in the US journal Proceedings of the
National Academy of Sciences. The research team, led by Steven Hecht Orzack, president and senior research scientist at the Fresh Pond Research Institute in Cambridge, England, evaluated what they called "the largest dataset ever assembled" on the sex ratio from conception to birth.
It was the first to their knowledge to include data from 3- to 6-day-old embryos created in fertility clinics, induced abortions, foetuses that had undergone screening such as amniocentesis for chromosomal abnormalities, and US census records of foetal deaths and live births.
They concluded that the sex ratio at conception was virtually 50-50. However, more male foetuses have genetic abnormalities and die during the first week or so. Then female mortality is greater for at least 10 to 15 weeks, followed by greater male mortality from the 28th to 35th week. In all, more female foetuses are lost during pregnancy.
In 2014, researchers from England's University of Exeter had reported in the British journal BMC Medicine that male foetuses were about 10 per cent more likely to be miscarried late in term.
They had evaluated more than 30 million births worldwide. Famine, interestingly enough, seems to favour the birth of females. In 2012, research done in the United States showed that the proportion of female births abruptly increased during and in the aftermath of the severe famine in China between 1959 and 1961.
The findings supported the hypothesis that women in poor condition are more likely to give birth to daughters. Although the reason is unclear, one theory is that female foetuses are nutritionally less demanding on mothers-to-be.
<<Print>>