Nearly All Infants Lose Weight After They’re Born. So Why Do Hospitals Focus on It So Obsessively?

Woman reaching towards baby hovering on the light side of a fulcrum balance scale.
Doris Liou

It’s ridiculous how many expectations and conventions exist around childbirth and parenting. But over the course of writing two books about this time of life, I’ve come to appreciate that there are some things you simply can not anticipate: things people don’t talk about, sometimes because they are sad, sometimes uncomfortable, sometimes just plain weird. I think we should talk about them—and, more than that, we should use data to really understand them. (I’m an economist; I love data). Acknowledging the data can often relieve a lot of the pressure on parents, either by reflecting experiences that feel isolating in the moment or by presenting us with a greater range of choices than we might have thought we had. In that spirit, I have a series in Slate this week about how data helps illuminate childbirth and parenting’s most underdiscussed topics.

My first child, Penelope, was born at 6 a.m. As a result, our insurance covered two nights in the hospital. On our second night—when my husband had gone home to rest and prepare the house for our return—the nurses took Penelope for some tests and returned her at 2 a.m. I was sleeping. The nurse switched on the light and rolled the bassinet in; in addition to Penelope, the bassinet had a little sign: Breastfeeding Only.

“We weighed her,” the nurse said, “and she’s lost 11 percent of her body weight. Our limit is 10 percent, so you’ll have to start supplementing with formula. If you don’t, you probably won’t get to take her home tomorrow.” I felt rising panic—not take her home?—and also some confusion. 10 percent versus 11 percent? These seemed pretty similar—was that one percent really enough to prevent an otherwise healthy baby from coming home?

Obviously, you want your baby to thrive, and weight is an important metric. But many new parents are not expecting the tremendous focus doctors and hospital staff place on infant weight gain or loss. If you have happily given birth to a healthy baby after a relatively uneventful delivery, the vast majority of your hospital conversations will now revolve around the baby’s feeding and weight. That might sound like a fine idea, but remember this is not a moment you are at your most laissez-faire. When you’re just postpartum and trying to breastfeed for the first time, it can be incredibly tense. It can feel like you are failing—you did such a great job growing this baby inside you, and now that it’s out, you are a failure. (You’re not!! That’s just how it feels.)

Here is the first thing to know: nearly all infants lose weight after birth, and those who are breastfed lose even more. The mechanisms for this are well understood. In the womb, your baby is getting nutrients and absorbing calories through the umbilical cord. Once the baby is out, he has to figure out how to eat. It is complicated (for both of you), and in the first few days, you won’t yet have a lot of milk.

Infant weight is monitored pretty carefully in the hospital. Every 12 hours or so, they’ll weigh the baby and possibly come back to report change in weight to you. Broadly, the reasons for weight monitoring are good ones. Weight loss is not an issue in and of itself, but excessive weight loss can indicate a problem with feeding—that breastfeeding isn’t working successfully, for example. This can be a clue that newborns aren’t getting enough liquid, which puts them at risk for dehydration. Dehydrated babies may then struggle more to feed, and you get a downward spiral. In principle this can have severe consequences, but these are rare.

Monitoring weight is about catching possible problems early, when you can fix them, and effective monitoring requires understanding how much weight newborns typically lose. Generally, we want to consider something a problem if it’s way outside the normal range. If most babies lose 10 percent of their weight, then we shouldn’t worry when that happens. Nothing in biology tells you that a baby losing 10 percent of its birth weight is a trigger for problems.

Figuring out the range of normal newborn weight loss requires data that, until recently, hasn’t been that easy to come by. In 2015, however, a set of authors published a really nice paper in the journal Pediatrics that used data from hospital records on 160,000 births to graph out the weight loss among breastfed infants in the hours after birth.

Weight loss percentages of newborns based on hours after birth.

The graph below shows a version of this study’s results for breastfed babies who were born vaginally. The horizontal axis shows infant age in hours; the vertical axis shows the percentage weight loss. The lines indicate how much this varies. The top line, for example, shows the weight loss path over time for the baby at the 50th percentile of weight loss.

From these figures, you can read both the average weight loss and the range. For example, at 48 hours, the average infant born vaginally has lost 7 percent of body weight, and 5 percent of infants have lost more than 10 percent. For at least some infants, weight loss continues through 72 hours.

On average, babies born by caesarean section lose a bit more weight than those born vaginally, likely because breast milk is usually a bit more delayed after a C-section. Babies who are formula-fed typically do not lose much at all (since formula doesn’t need time to come in).

When I had Penelope, the rule in the hospital was that if babies lose more than 10 percent of their body weight, you supplement. But you can see from the graphs that whether this is a reasonable cutoff depends tremendously on when the measurement is taken and the baby’s particular circumstances. At 72 hours, 10 percent weight loss is inside the normal range. At 12 hours, it would be a serious outlier. The authors of this paper created a very nice website where you can enter the time of birth of your child, method of birth, method of feeding, birth weight, and current weight, and learn where your baby is in the distribution.

If you do find, as I did, that your infant has gone over the weight loss limits, what should you do? Typically, hospitals will recommend supplementing with formula for a short period. Supplementation would rarely be recommended before 48 or 72 hours, so it’s useful to pay attention to your baby’s weight before that. If she’s losing weight quickly, trying to figure out why may make sense.

A final note: The major concern about weight loss is that it signals dehydration. But this is also something you can monitor directly. If your baby is peeing with some frequency and does not have a dry tongue, there’s a very good chance he’s not dehydrated. Conversely, if you do see these signs, supplementation may be a good idea even if there isn’t too much weight loss.

The extensive focus on weight and feeding is enough to really scare a lot of new parents—myself included. But the data here should be reassuring. Some pretty substantial weight loss is totally normal, even expected. So don’t be surprised, and don’t panic. Even when you’re trying and failing to feed an infant at 2 a.m.

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