Seem like everyone you know is making the "It's a boy!" announcement? It's not just you — it's statistics.
Since the 17th century, scientists have noticed a slightly tilted sex ratio at birth: 51 percent of babies born are boys. The long-held belief is that gender is determined at conception, but a group of biologists decided to probe further into this assumption.
Researchers from Harvard, Oxford, Fresh Pond Research Institute and Genzyme Genetic collected 140,000 embryos created in fertility clinics and an additional 900,000 from fetal screening tests. Combined with 30 million records from live births, miscarriages and abortions, the massive amount of data made this the largest investigation of its kind.
What's surprising is what researchers didn't find. There was no imbalance of male and female embryos at the time of conception. The takeaway? Sometime during pregnancy, the sex ratio becomes skewed. During the first week of pregnancy, more male embryos actually die than female.
"When that settles out, it looks like there starts to be an excess of female mortality," says researcher Steven Orzack. "And in the third trimester, as has been known for a long time, there is a slight excess of male mortality."
When all is said and done, more female fetuses are lost during pregnancy, and that's why we have more baby boys. You can learn more about the findings in the Proceedings of the National Academy of Sciences.