Why do male babies generally have poorer health outcomes than female babies? Researchers from the University of Adelaide in Australia investigate. They publish their findings in the journal Molecular Human Reproduction.
Previous research has shown that boy babies grow faster in the womb, having a greater body length and weight than girl babies at birth. Some researchers have suggested this shows that the male placenta functions more efficiently. However, there seems to be a trade-off, as boy babies are consequently left with less reserve placental capacity to draw upon than girls in the event of adverse health conditions.
As a result, male babies are more at risk of undernutrition, which restricts growth and lowers birthweight, and puts them at greater risk of cardiovascular disease in adulthood.
In the new study, the researchers wanted to investigate whether there was a difference between boys and girls in the type and pattern of genes expressed.
Senior author of the paper Prof. Claire Roberts, leader of the fetal growth research priority for the Robinson Research Institute at the University of Adelaide, says their study has found "undeniable genetic and physiological differences" between boys and girls that go beyond just the development of their sexual characteristics.
"We've known for some time that girls are clearly winning in the battle for survival, with markedly better outcomes for female babies for preterm birth, stillbirth, neonatal death, and other complications after birth, such as macrosomia (a baby that weighs more than 4-4.5 kg or 8 pounds 13 ounces at birth). Male babies generally grow faster and bigger than females. This occurs in both the animal and human worlds, but until now we haven't really understood how or why."
'Girls more likely to adopt risk-averse strategy toward development, survival'
Analyzing 300 placenta samples, the researchers found more than 140 differences in gene expression between the male and female samples.
In particular, there were differences between male and female samples in genes encoding the LH and hCG hormones that promote placental growth. The researchers say this shows that female fetuses invest more in "extra-embryonic tissue development" than males.
According to the new study, female fetuses invest more in "extra-embryonic tissue development" than males.
This finding supports previous research, suggesting that boy babies invest resources in embryonic tissues (body growth and development) and so invest less in the extra-embryonic tissues that baby girls are able to call upon in the event of pregnancy complications or disease.
"Our results suggest that there is a distinct sex bias in the regulation of genes in the human placenta," says lead author and University of Adelaide PhD student Sam Buckberry. He continues:
"We found that with female babies, there is much higher expression of genes involved in placental development, the maintenance of pregnancy and maternal immune tolerance. This suggests that girls are more likely to adopt a risk-averse strategy towards development and survival, and it goes some way to explaining the differences in male and female development in the womb."
The researchers believe that their findings are important in extending knowledge around differences in health outcomes for boy and girl infants as this is the first to observe widespread sex-biased gene expression.
Recently, Medical News Today reported on a study that found the placenta is not as sterile an environment as previously thought, but instead harbors a low but diverse population of bacteria, similar to that found in the human mouth.