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Do Giant Babies Grow Into Giant Adults?Birth size and its consequences.

By Daniel EngberUpdated Wednesday, Dec. 21, 2005, at 5:59 PM ET

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An Oklahoma woman gave birth to a 14-pound, 3-ounce baby girl on Dec. 16. Hospital workers say the newborn is already wearing diapers and clothing designed for 9-month-olds. Do giant babies like this one turn into giant adults?

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Yes. There's no way to predict exactly how big this enormous infant will become, but studies have shown a linear correlation between birth weight and adult size (as measured by the body mass index). We also know that the length of a baby is associated with its eventual height and weight. In other words, heavy babies tend to grow up fat and long babies tend to grow up tall.

Related in Slate

Liza Mundy explained why test-tube babies tend to have low birth weights. Emily Bazelon reviewed the diaper-free baby movement. In April, Slate published an excerpt from Steven D. Levitt and Stephen J. Dubner's Freakonomics on the prognosis for babies with distinctively black names.

Studies have also shown that bigger parents have bigger babies, which in turn end up as bigger adults. (Parents who were born heavy themselves are also more likely to have large babies.) This should come as no surprise: Children inherit their parents' body types both via genetics and shared experience. But the data show a connection between birth weight and eventual BMI that can't be explained by the parents' size or lifestyle. Identical twins, for example, seem to end up at sizes that reflect the difference in their initial birth weights.

On the other hand, a baby that weighs less will likely grow into an adult with a lower BMI. (She'll also have less of a chance of getting breast cancer.) That doesn't mean you should hope for a small baby: Lower birth weight has been associated with stroke, coronary heart disease, and other problems. To make matters worse, when an especially little baby grows up, the fat she does have tends to accumulate in her trunk—another risk factor for cardiovascular illness. (There's some controversy over evidence that smaller babies end up with higher blood pressure.)

You can also find connections between birth weight and cognitive abilities. Larger babies tend to score higher on IQ tests when they grow up. A study from the University of Pennsylvania found that on average, each additional pound of baby fat yields four more months of schooling and a 7 percent increase in earnings.

Bonus Explainer: What makes a baby big? In some cases, diabetes. If an expectant mother can't metabolize sugar properly, her fetus may start producing extra insulin. The insulin functions as a growth hormone that makes the baby bigger and increases its chances of growing up overweight. Mothers who put on a lot of pounds during their pregnancies are also more likely to have larger babies.

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Explainer thanks Matthew Gillman of Harvard Medical School.