Girl or boy: For expecting parents, it’s a classic question. For scientists studying human demographics, it’s a head scratcher.
Statistics seem to favor boys. On average, for every 105 boys born, only 100 girls are born. Scientists have credited the difference to more male embryos being conceived. But that’s not true, a new study suggests.
Researchers from the United States and the United Kingdom analyzed data from roughly 35 million embryos, fetuses and births and conclude that at conception, the ratio of male to female embryos is roughly 50-50. The sex ratio may skew slightly in favor of females in the first week or so after conception. But then it flips to favor males starting at 10 to 14 weeks, possibly explaining why slightly more boys than girls are born. The team reports the findings online March 30 in the Proceedings of the National Academy of Sciences.