By Kathleen Doheny
After analyzing more than half a million births in Australia, researchers said the baby's gender could be linked to the health of both mother and child.
"The sex of the baby has a direct association with pregnancy complications," said study first author Dr. Petra Verburg, of the Robinson Research Institute at the University of Adelaide in Australia.
Boy babies were more likely to be born early, which sets up infants for more health problems. Also, women carrying boys were slightly more likely to have diabetes during pregnancy (gestational diabetes), and pre-eclampsia, a serious high blood pressure condition, when ready to deliver, the study authors said.
Although it isn't totally clear why this is so, "there are likely to be genetic factors," Verburg said.
The findings ring true, said Dr. Querube Santana-Rivas, a neonatologist at Nicklaus Children's Hospital in Miami, who wasn't involved in the study. She said she sees the differences in her own practice.
"Male gender is a risk factor for a lot of the complications right after birth, especially in the premature population," Santana-Rivas said.
The results also echo some findings from previous studies, Verburg said. A potential explanation is that the placenta, the organ that nourishes the developing fetus, is different in boys and girls.
"The placenta is critical for pregnancy success, and it is an organ that technically belongs to the baby, so it is genetically identical to the baby," said study co-author Claire Roberts, another researcher at the Robinson Research Institute.
In previous research involving normal pregnancies, Roberts' team found sex differences in the expression of 142 genes in the placenta. The researchers said that defects in how the placenta develops and works are linked with pregnancy complications.
For the new study, Verburg, Roberts and colleagues evaluated more than 574,000 Australian births from 1981 through 2011.
Compared to girls, boys had 27 percent higher odds of preterm birth between 20 and 24 weeks' gestation; 24 percent greater risk for birth between 30 and 33 weeks; and 17 percent higher odds for delivery at 34 to 36 weeks, the study found. Full-term birth is between 39 and 41 weeks, according to the American College of Obstetricians and Gynecologists.
Moreover, gestational diabetes was 4 percent more likely in women carrying boys, and pre-eclampsia at term was 7.5 percent more likely with boys, the researchers said.
However, women carrying girls had a 22 percent higher risk of getting pre-eclampsia early in pregnancy, requiring preterm delivery, the study found.
Still, the research merely shows an association between gender and birth complications, not a cause-and-effect relationship. The findings shouldn't alarm mothers-to-be, no matter what the sex of their unborn child, said Roberts.
The advice, for now, is the same as for all women who become pregnant, Roberts said. That means eating a good diet and attempting to maintain a healthy weight before conceiving.
"Even if the pregnancy was unplanned," Verburg said, "there is still a window of opportunity for a woman to reduce her risks for pregnancy complications." A woman can stop smoking, not drink alcohol and stay physically fit, she said.
Santana-Rivas agreed. She said the take-home message from the new study is for women to be aware of the potential risks "and to get good prenatal care."
Depending on what future research finds, prenatal programs for pregnant women might one day vary based on whether they are carrying a boy or a girl, the researchers said.
The study was published online July 11 in PLOS ONE.