Why being a boy is a risk factor among premature births

Preterm birth entails a risk of complications like low blood pressure, brain hemorrhaging, lung immaturity, and in the long term neurological and cognitive handicaps. The risk is greater the more preterm the birth is: complications primarily affect the most premature children, born before the 28th week.

Emma Elsmén Steen has examined sex differences in this morbidity through journal and registry studies and work in the clinic and the laboratory.

"It's well known that boys are more in the risk zone than girls. I wanted to delve deeper into this matter and above all see what happens during the first critical days," she says.

In a study of journals for more than 200 premature births, she was able to show that boys are sicker than girls during this first period. For instance, they more often needed respiratory help from the very beginning, and also more often needed help to keep their blood pressure up. Despite this treatment, boys still had lower blood pressure after birth and more often developed chronic lung disease.

Another study, based on data in a national database, focused on the occurrence of complications in more than one million births. Pregnancies with male fetuses turned out to have a greater risk of being affected by all of the complications that were studied: pregnancy toxemia, excessive amniotic fluid, infection, separation of the placenta, and premature loss of amniotic fluid.

It was only in the cases of "excessive amniotic fluid" and toxemia with birth before the 32nd week that the risk was higher if the fetus was a girl.

"'Preeclampsia' -- a form of toxemia -- is a disease that we still know very little about. It's possible that the preeclampsia that occurs early in the pregnancy, and primarily affects mothers with girl fetuses, is another disease than the one that occurs later in the pregnancy and primarily affects women with boy fetuses," says Emma Elsmén Steen.

Research on sex differences in morbidity during the fetal period has experienced an upswing in recent years. On the one hand, more knowledge in this field can yield a greater understanding of morbidity in newborns in general and, on the other hand, we are gaining greater insight into the importance of sex in a number of medical contexts.

Emma Elsmén Steen believes that the greater morbidity among premature boys can be due to the fact that male fetuses mature later. This can mean that they have a poorer ability than females to react to stress, respond to inflammation, and regulate their blood pressure. In full-term pregnancies, boys have had more time to catch up. Here, too, boys run a greater risk of sickness and dying than girls do, but the difference between the sexes is not as great.