(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.
"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.
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