Kids' Health

What A Baby’s Birth Weight Really Tells You

Why baby birth weight matters, and when it doesn't.

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Get ready, dads-to-be: You’re about to be asked a million times over how much your baby weighed at birth. This is one of the most common questions new parents get from friends, relatives, even strangers on the street.

Why the obsession with birth weight? “In general, it can be a good indicator of the baby’s health,” says Dr. David Hill, author of Dad to Dad: Parenting Like a Pro. So in other words, what all those inquiring minds really want to know is whether your baby was born healthy.

But while birth weight is a decent barometer of health, the number is not a perfect gauge. A tiny baby could be clear-eyed and rosy-cheeked, while a normal-sized newborn may have a host of health issues. That said, there are a few general guidelines that can give you some peace of mind and help you know what to expect.

Baby Birth Weight and Gestational Age

First off, birth weight is largely dependent on gestational age, meaning how much time the child spends in the womb. For babies carried full-term, the average weight is between 6 and 9 pounds. But for those born either early or late, the averages are slightly different.

“In most cases, if a baby is born prematurely, he is going to be smaller, whereas if he is born late, he’ll be larger,” Hill says. “So in assessing birth weight, the first thing we do is adjust for gestational age. A baby might be small in the absolute sense because he was born at 36 weeks, but if his size is average for a 36-week term, then he is considered average.”

Because premature babies don’t get the full benefit of developing in utero, they run a higher risk of health complications, such as jaundice, anemia, respiratory distress, or heart issues, often requiring a stint in neonatal intensive care unit. But again, just because a preemie is smaller than a full-term baby doesn’t automatically mean they’re less healthy.

Baby Birth Weight and Genetics

As for babies who are carried to term, even they sometimes fall outside the 6- to 9-pound range, for any number of reasons. Sometimes it’s just genetics — perhaps both parents are petite, or maybe they’re tall and broad. More often, though, a lower- or higher-than-average birth weight is a reflection of the mother’s health.

When a newborn weighs less than 90 percent of babies of the same gestational age (putting them in the 10th percentile), they are considered “small for gestational age,” or SGA. Oftentimes, SGA is the result of the fetus not getting the full supply of nutrients or oxygen needed for normal organ and tissue development, which can stem from conditions affecting the mom. For instance, Hill says pregnancy-induced hypertension can reduce a baby’s weight. So can heart, respiratory, or kidney issues. Smoking and drinking during pregnancy can also lead to SGA. “Hopefully, most women know that by now,” says Hill. “But we can’t say enough that doing either during pregnancy is a horrible idea.”

On the other end of the spectrum, if a baby weighs more than 90 percent of newborns of the same gestational age (they’re in the 90th percentile), they are considered “large for gestational age,” or LGA. This is can be caused by the mom’s obesity or excess weight gain during pregnancy. Most often, it’s also tied to gestational diabetes.

Baby Birth Weight and Diabetes

“Mothers who have gestational diabetes or even just glucose intolerance can cause elevations in their babies’ blood sugar,” Hill says. High blood sugar, in turn, prompts the child’s body to crank out insulin, resulting in a bigger, chunkier newborn. “These babies can be so big that it causes problems with delivery or even necessitates a C-section,” Hill says. C-sections are riskier for both mom and baby than vaginal births.

“There are definitely some health issues in the newborn period associated with both SGA and LGA,” Hill says. “Surprisingly, those issues are the same: maintaining healthy blood sugar. Underweight babies don’t have adequate fat stores to keep them warm or their blood sugar up.” If their blood sugar drops too low, it can be life-threatening, he adds, and they must be given nutritional supplements or dextrose through an IV to keep their blood sugar up until they stabilize and begin feeding normally. Similarly, when an LGA baby is born to a diabetic or glucose-intolerant mom, they end up generating too much insulin shortly after birth, causing their blood sugar to drop and requiring intervention.

Hill says LGA babies often lose weight pretty quickly after they are born. While most normal-weight babies will lose about 10 percent of their birth weight before regaining it, LGA babies can drop even more. “If an average-weight baby is losing more than 10 percent in the first week, we’d worry he is not getting enough nutrition so we’d keep a close eye,” Hill says. “But if the mom had diabetes, an LGA baby losing more than 10 percent of his body weight isn’t so alarming—he is likely just coming back to the weight he was supposed to be in the first place.”

Because both SGA and LGA carry health risks, “if you are planning on a home birth, make sure that whoever is assisting you is aware of the baby’s weight as predicted by an ultrasound and is prepared to deal with any associated issues,” says Hill.

Wherever your newborn lands on the spectrum—SGA, a tad small, average, a bit big, or LGA—they have a good shot at becoming a bouncing, healthy, happy baby. Just make sure you commit that birth weight to memory—and gear up for a whole new slate of weight questions as your child grows.

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