How big is my baby going to be?

Your baby's size depends on a number of things, including his or her parents' size (mother's size more than father's), a mother's preexisting medical conditions (chronic hypertension and kidney disease are risks for small babies), medical complications during pregnancy (gestational diabetes is a risk for having a large baby and preeclampsia a risk for a small baby), medications used in pregnancy, and exposures to toxins like cigarette smoke. While there are lots of reasons to follow a healthy diet before and during pregnancy, diet has less influence on a baby's size than you might imagine, especially in areas of the world in which calorie intake is not markedly limited.

The most important influence on birth weight however is also one of the most difficult to predict: the length of your gestation (i.e. the amount of time you carry the baby before you give birth). The longer he or she is in your womb, the more time the baby has to grow. And the last weeks of pregnancy are when the baby puts on fat and muscle. Since babies born prematurely don't have the time to do that, premies are generally smaller.

But sometimes even babies born at term are small and, conversely, babies born early are larger than expected. When we speak of size, we often compare weight to what is expected for a given point in pregnancy. At full term, babies weighing less than 2,500 gm (5 lb, 8 oz) and more than 4500 gm (9 lb, 14 oz) are, respectively, unusually small or big. In addition to influencing your child's immediate health after delivery, birth weight has been linked to later childhood and adult health issues. According to some studies, small babies are more likely to have heart disease and big babies may be more likely to have diabetes and be obese later in life. Finally, having a very big baby also puts a woman at risk for needing cesarean delivery.

As I mentioned in last week's blog, your baby's size can be estimated via an ultrasound, but such estimates are just that, estimates. Anytime patients and doctors make decisions based on ultrasound measurements they need to do so with the understanding that actually birth weight can vary by 10% or more from what's predicted.

What would you like to know about baby size and its affect on either the mom or the baby?

Jeffrey Ecker, M.D., is an attending perinatologist (high-risk obstetrician) at Massachusetts General Hospital. He is also an Associate Professor at Harvard Medical School.

Eat, Play, and Be Healthy
Parents are often bombarded with new information on children's nutrition, and as a result, the most important dietary considerations often get lost in the mix. From Harvard Medical School, Eat, Play, and Be Healthy offers guidance on healthy eating through the various stages of children's lives, from infants to eight-year-olds. From breastfeeding to school lunches, get the tools to put your child on the path to a healthy adulthood.

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