(CBS Local) — Physical or mental stress during pregnancy may determine the baby’s gender, according to a study published Monday.

The National Academy of Sciences study found fewer boys were born to stressed out moms-to-be. Male fetuses are more vulnerable and long term stress could pay a role in pregnancies, the report’s lead author said.

“The womb is an influential first home,” said lead author Catherine Monk, director of the women’s mental health program in obstetrics and gynecology at NewYork-Presbyterian/Columbia University Irving Medical Center.

The study found four boys were born for every nine girls when the mother had high blood pressure while moms who were psychologically stressed had two boys for every three girls. Nature typically stacks the gender pool at 105 males to 100 females.

“Other researchers have seen this pattern of a decrease in male births related to traumatic cataclysmic events,” Monk told CNN. “One of them being President Kennedy’s assassination and the other being the September 11 terrorist attacks in New York City.”

The risk of pre-term birth also went up with higher stress levels.

While the study did not explore exactly how stress impacts pregnancy, others have pointed to the role of the stress hormone cortisol, which is part of the body’s “flight or flight” alarm system and is meant to dissipate once the danger is over

Chronically high levels of cortisol can increase blood pressure and make an individual more prone to illness, CNN reported.

Researchers emphasize that stress is a normal part of life and during pregnancy. They say getting plenty of sleep, eating healthy and exercising can help minimize the effects of stress.

They also say social support can reduce the risk of premature delivery and increase chance of having a male baby.

“The support could be from family and friends,” Monk said. “It could be a sense of belonging in a religious community. It’s the sense of social cohesion and social connectedness which research suggests is a buffer against the experiences of stress. It means you take a break from it.”