New study by Dutch researchers suggests that bigger babies are born to older women, while younger women have smaller babies.
Previous studies hinted at the link between a mom’s age and her baby’s birth weight, as well as possible health consequences that may arise when babies are too small or too big. Babies who grow smaller than normal in the womb are at risk for increased birth complications, and are more likely to have diabetes and heart disease in adulthood. Babies who grow larger in the womb are more likely to become obese later in life.
The new study says past findings could be particularly relevant as the age at which women are giving birth is still growing in the Western world.
According to a 2010 study by the Pew Research Center, the percentage of US babies born to women older than 35 grew from 9 percent in 1990 to 14 percent in 2008.
Rachel Bakker of the Erasmus Medical Center in Rotterdam, the Netherlands and her colleagues studies more than 8,500 Dutch women who gave birth between 2002 and 2006.
In the study, the average newborn weighed 7.7 pounds. Researchers defined “small” as a baby weighing 5.5 pounds or less and a “large” baby as weighing 10 pounds or more.
Based on the childbirths in the study, about one in 20 newborns are born small, and an additional one in 20 are born large.
Compared to women between the ages of 30 and 35, those under 25 tended to be more likely to have small babies. About 4 percent of 30-to-35-year-olds had small newborns, while 7 percent of mothers under 20 had smaller babies.
The study found that older mothers were more likely to have large babies. The risk of having a large baby went from 3 percent in very young women, to almost 6 percent in those between 30 and 35, to roughly 10 percent in mothers over 40 years old.
In the young mothers, the link between age and risk of delivering small babies was mostly due to social factors (including ethnicity, education level, and how many times a woman had given birth before) and lifestyle factors (like diet, smoking and alcohol use).
In other age groups, social factors could also explain why younger women tended to have smaller babies. But none of the factors could explain why the risk of having a large baby went up in older women.
The findings could also suggest that other factors in women’s bodies might be playing a role, but the team said it is unclear right now what those factors may be.
Bakker said more research is needed to clarify what factors are associated with mother age and birth weight, and to find the range of potential health effects.
The findings do not mean a mother’s age alone causes her baby to be born large or small, according to the team of researchers. Other factors could also affect the newborn’s size, including mother’s weight, tobacco and alcohol use, and how many other childbirths the mother went through.
In the meantime, there isn’t enough information available “to advise women about the most optimal age to have children,” Bakker to Reuters Health in an email.
The study was published in January in the British Journal of Obstetrics and Gynecology.
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