(CNN)Moms-to-be who undergo physical or mental stress during their pregnancies are less likely to have a boy and may also have a higher risk of preterm birth, according to a study published Monday.
A stressful pregnancy reduces the chances of having a boy, a study shows
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"The womb is an influential first home," said lead author Catherine Monk, director of women's mental health in OB/GYN at NewYork-Presbyterian/Columbia University Irving Medical Center.
Even though the sex of a baby is dictated at conception, pregnancies with boys are more vulnerable to mishap, she said.
"We do know that males are more vulnerable in utero, and presumably the stress in these women is of a long-standing nature," Monk said.
Nature typically assures there are an average of 105 boys born for every 100 female births; after all, males were more likely to die from accidents or fighting wild game.
But in this study, women who had higher blood pressure and other signs of physical stress had four boys for every nine girls (ratio 4:9); while moms who were psychologically stressed had two boys for every 3 girls (ratio 2:3). All of the women had healthy pregnancies.
"Other researchers have seen this pattern of a decrease in male births related to traumatic cataclysmic events," Monk said. "One of them being President Kennedy's assassination and the other being the September 11 terrorist attacks in New York City."
Pregnant women who were physically pressured were also more likely to give birth prematurely than unstressed mothers. However, mentally tense moms-to-be had more birth complications, such as longer labor, than the moms with physical stress.
Yet when the expectant mothers received social support, such as having someone they could talk to or count on for help with their problems, the risk of premature delivery disappeared -- a surprising discovery, Monk said.
Even more surprising, the more social support a mother received, the greater the chance she had 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."
Social support is critical for building confidence in a new mom, said Dr. Christina Penfield, an assistant professor of maternal-fetal medicine at New York University Langone Medical Center, who was not involved in the study.
"Pregnancy is a pivotal moment in women's lives coinciding with a redefinition of self, family, and community," Penfield said. "It is therefore not surprising that several studies have demonstrated that when we provide social support programs to pregnant women we see improvements in their psychosocial outcomes."