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Here's a notable fact; 51 percent of the babies born in the United States are male. Why that ratio isn't 50-50 is a mystery since that's what basic biology would predict. NPR's Richard Harris reports on a new study with some unexpected clues.
RICHARD HARRIS, BYLINE: This tilted sex ratio at birth has been known for centuries, and scientists have been puzzling about it ever since. The long-held assumption is that this imbalance starts at the very moment of conception - that more males are conceived than females. Biologist Steven Orzack says that was the entrenched conventional wisdom.
STEVEN ORZACK: There were a number of people who have said, hold on, there's some biases. We don't have much business saying anything about the sex ratio at conception. But for the most part, people didn't listen to them.
HARRIS: Orzack, who works at a small research outfit in Cambridge, Mass., called the Fresh Pond Research Institute, decided to dig into this question with a few colleagues. They collected information from more than 140,000 embryos that had been created in fertility clinics, almost 900,000 samples from fetal screening tests, like amniocentesis, along with tens of millions of records from abortions, miscarriages and live births. Most of this came from the U.S. and Canada, not countries like China, where parents more often abort female fetuses.
ORZACK: This is the largest compilation of data for this particular kind of investigation that's ever been put together.
HARRIS: And, as they report in The Proceedings of the National Academy of Sciences, they did not see the long-assumed difference between male and female embryos at the time of conception.
ORZACK: The best estimate we have is that it's even-steven - 50 percent males, 50 percent females.
HARRIS: So that must mean that the skewed sex ratio at birth happens during pregnancy. Looking deeper, Orzack and his colleagues found that in the very first week of pregnancy, more male embryos died, possibly as a result of serious chromosomal abnormalities.
ORZACK: The first trimester after that, when that settles out, it looks like there starts to be an excess of female mortality. That's a new fact. Some people had hinted at it, but we're the first people to demonstrate it. That lasts until about week 15, 16 or so, then things settle out. And in the third trimester, as was known for a long time, there's a slight excess of male mortality.
HARRIS: When you put all this together over the course of pregnancy, more males are born because more female fetuses are lost during pregnancy.
ORZACK: That's completely opposite to what had been believed for a long time.
HARRIS: To Eugene Pergament, an obstetrician at Northwestern University, the 1-to-1 sex ratio at conception is not the most interesting part of this paper.
EUGENE PERGAMENT: It's always sexy to talk about sex.
HARRIS: But he says that the research's greatest asset is that it sheds light on what's going on during early pregnancy, a time when scientists have very little understanding of what's happening within a developing embryo.
PERGAMENT: I think it will eventually have greater consequences and significance in our understanding normal and abnormal human development.
HARRIS: The author, Steven Orzack, says he's hoping all sorts of researchers can turn his observations into insights. Richard Harris, NPR News.
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