If you don't know your baby's sex, the odds are you're having a boy since 51 percent of babies born in the United States are male. Science had no idea why, until now.
Fifty-one percent of the babies born in the United States are male and it's been a long-held belief that that imbalance starts at conception with more males conceived than females. But according to biologist Steven Orzack at the Fresh Pond Research Institute in Cambridge, Mass., who has been studying the birth-sex ratio, there is no hard data to support this idea.
Orzack, along with scientists and researchers from Harvard, Oxford and Genzyme Genetics, collected information from 140,000 embryos created in fertility clinics, 900,000 samples from fetal screening tests like amniocentesis and 30 million records from abortions, miscarriages and live births, mostly from the U.S. and Canada. According to their study, published in the Proceedings of the National Academy of Sciences there's no evidence of more boys being conceived than girls. It's pretty much even at conception: 50 percent males and 50 percent females.
So how can there be more baby boys born than girls?
Instead, it appears that the skewed sex ratio happens in pregnancy: The researchers found that although more male embryos died in the very first week of gestation, possibly as a result of serious chromosomal abnormalities, there was a higher rate of female mortality later in pregnancy.
Male fetuses were once thought to be more fragile than females, but the study disproved that theory as well. According to Orzack, these long-held ideas of about the sex ratios weren't based on a great deal of evidence. "It's 180 percent different than what scientists had believed for a long time," Orzack says.
While odds are, if you're a betting woman on your way to a gender reveal party, you'll be seeing blue, the information gleened from the study holds more value for science than just info on sex: It's a little more information about what's happening to the developing embryo during early pregnancy, a time that science still doesn't wholly understand.