SOURCES: Steven Orzack, Ph.D., president and senior research scientist, Fresh Pond Research Institute, Cambridge, Mass.; William James, Ph.D., associate research fellow, department of biometry and Genetics, University College London, England; March 30, 2015, Proceedings of the National Academy of Sciences
MONDAY, March 30, 2015 (HealthDay News) -- Many scientists believe human conception produces more male than female embryos, with male embryos less likely to survive.
But, a new study suggests that equal numbers of males and females are conceived and female embryos are less likely to survive.
"It looks like more females die during pregnancy than males," said Steven Orzack, a senior research scientist with the Fresh Pond Research Institute in Cambridge, Mass. "People have long thought the opposite was true. An important fact that people thought was reasonably well-demonstrated is probably incorrect."
The findings aren't definitive, and a researcher not involved with the study who has focused on the subject believes this new hypothesis is wrong. But Orzack said he and his colleagues are on solid ground with statistics that challenge long-held assumptions about human development.
According to Orzack, many scientists have believed that more males are conceived than females. Slightly more males are born than females, he said, and then females begin to outnumber males as people grow older because men die at younger ages. By the time people are seniors, women significantly outnumber men.
"It's important to study male-female differences in the womb because they underlie, in part, the profound differences we see between males and females at birth and thereafter," Orzack said.
In the new study, researchers looked at the records of more than 30 million embryos, fetuses and babies that survived past childbirth. Almost all the records were from census records of live births. But the researchers also examined tens of thousands of records of the genders of embryos and fetuses that were the product of fertility treatments, underwent testing during pregnancy or were aborted.
The researchers found evidence that approximately equal numbers of males and females are conceived, but female embryos are more likely to die than male embryos during the first trimester. Males are more likely to die during the third trimester, the researchers said.
Could modern obstetrics be affecting the ratio of boys to girls? The statistics for the new study go back for decades, even to the 1930s, so it's hard to know for sure, Orzack said. It's possible, he said, that some parents learn of their unborn child's gender and prefer to abort if the child isn't a boy. Still, "I don't think that has a huge effect on the ratio we're talking about," he added.
William James, a longtime researcher into gender ratios in the womb and an honorary research fellow at University College London, dismissed the new study as "largely invalid because much of the data they [the study authors] cite is not of healthy women."
Study author Orzack responded that "there's no reason to think" that the health of the women has anything to do with the ratio of males to females. As for the health of the women in the study, he said it includes census data on almost all live births and fetal deaths plus numbers from other women (such as those who underwent fertility treatments and abortions) who may be in good health. "This is the best kind of data that we can get," he said, although "it's not perfect."
What's next? "We'd like to pursue research that would help us understand why it appears that more females are dying than males" earlier in pregnancy but then reverses later, Orzack said.
The study appears in the March 30 issue of the Proceedings of the National Academy of Sciences.
For more about pregnancy, visit the U.S. National Library of Medicine.