Older mom, bigger baby

Prior studies have hinted at links between a mother's age and her baby's birthweight, as well as potential health consequences when babies are very small or very large. Babies who grow less than expected in the womb have a higher risk of birth complications, as well as diabetes and heart disease in adulthood. Very large newborns may be more likely to become obese later in life.

This could be particularly relevant as the age at which women are having children is still increasing in the Western world. For example, according to a 2010 study by the Pew Research Center, the percentage of US babies being born to women older than 35 went from 9% in 1990 to 14% in 2008.

Researcher Rachel Bakker of the Erasmus Medical Center in Rotterdam, and her colleagues studied 8,568 Dutch women who gave birth between 2002 and 2006.

The average newborn weighed around 3,5kgs. The researchers defined a "small" baby as one weighing 2,5kgs or less and a "large" baby as weighing 4,5kgs or more.

In total, one of every 20 newborns was small, and another one in 20 was large.

The risk of a large baby went from 3% in the very youngest women, to about 6% in 30-to-35-year-olds, to roughly 10% in moms over 40.

For the very youngest mothers, the link between age and the risk of delivering a small baby was mostly due to social factors (such as ethnicity, education level, and how many times a woman had given birth before) and lifestyle factors (such as such as diet, smoking and alcohol use).

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