Why Are More Baby Boys Born Than Girls?

Mar 30, 2015
Originally published on April 17, 2015 12:58 am

Scientists have found some unexpected clues that could help explain why 51 percent of the babies born in the United States are male.

It's been a mystery why that ratio isn't 50:50, since that's what basic biology would predict. But scientists have noticed a tilted sex ratio at birth since the 17th century.

The widely held assumption is that this imbalance starts at the very moment of conception — that more males are conceived than females.

"There were a number of people who said, hold on, we don't have much business saying anything about the sex ratio at conception," says biologist Steven Orzack at the Fresh Pond Research Institute in Cambridge, Mass. "But for the most part people didn't listen to them."

Orzack, along with colleagues from Harvard, Oxford and Genzyme Genetics, decided to dig into this question. They collected information from more than 140,000 embryos that had been created in fertility clinics, along with almost 900,000 samples from fetal screening tests like amniocentesis and 30 million records from abortions, miscarriages and live births. Most of these data came from the U.S. and Canada, not countries like China, where parents more often abort female fetuses.

"It's the largest compilation of data for this kind of investigation that's ever been put together," Orzack says.

And they now report in the Proceedings of the National Academy of Sciences that they did not see the long-assumed difference between male and female embryos at the time of conception.

"The best estimate we have is that it's even-steven — 50 percent males [and] 50 percent females," Orzack says.

So that must mean the skewed sex ratio at birth happens during pregnancy. Looking deeper, the researchers found that in the very first week of pregnancy, more male embryos died, possibly as a result of serious chromosomal abnormalities, which they also documented.

"When that settles out, it looks like there starts to be an excess of female mortality," Orzack says. "And in the third trimester, as has been known for a long time, there is a slight excess of male mortality."

When you put this all together, it turns out more males are born because more female fetuses are lost during pregnancy.

"That's completely opposite to what had been believed for a long time," Orzack says.

Explaining why more boys are born than girls is, of course, a catchy result. "It's always sexy to talk about sex," says Dr. Eugene Pergament, an obstetrics researcher at Northwestern University.

But he says the research's greatest contribution is that it sheds light on what's going on during early pregnancy. That's a time when scientists have very little understanding of what's happening within a developing embryo, and what external influences may be affecting its development and survival.

"I think it will eventually have greater consequences and significance in our understanding normal and abnormal human development," Pergament says.

Orzack says he's hoping all sorts of researchers can now turn his observations into insight.

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Transcript

DAVID GREENE, HOST:

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. Transcript provided by NPR, Copyright NPR.