No matter their size, newborn stomachs need frequent filling

mom breastfeeding baby

ALL DAY, ALL NIGHT Stomach size is just one of several factors that drive a newborn to feed.


I’m making my way through my third round of breastfeeding a newborn and taking stock of how things are going. Some aspects are definitely easier: My milk came in really quickly (a perk of being a repeat lactator), the fancy breastfeeding baby holds are no longer mysterious to me and I already own all of the weird pillows I need to prop up my tiny baby.  

But one thing isn’t easier this time around: the bone-crushing, mind-numbing exhaustion. Just like my other two, this sweet baby seems to eat all the time. All day. All night. Sometimes multiple times an hour, especially in the witching hours of the evening. This frequency got me curious about the biology of newborns’ stomachs. Just how small are they? Are they so microscopic that one can hold only enough sustenance to keep my newborn satisfied for a thousandth of a second?

Birth educators and medical professionals often use a marble to illustrate the size of a newborn’s stomach, a tiny orb that holds about 5 to 7 milliliters of liquid. But that small estimate has come into question. A 2008 review published in the Journal of Human Lactation points out that there aren’t many solid studies on the size of the infant stomach, and some of the ones that do exist come to different conclusions. Another review of existing studies concluded that the average newborn stomach is slightly smaller than a Ping-Pong ball and can hold about 20 milliliters, or about two-thirds of an ounce.

The question of stomach size at birth isn’t settled, and that may be in part because there is no one answer. Just as babies come in a variety of sizes, their stomachs may too. What’s more, size isn’t everything. Drinking ability, the stretchiness of the stomach and the speed at which food gets digested are all part of the equation, and those physiological skills may take a few days, or longer, to get sorted out. There’s evidence that brand new stomachs get better at relaxing after a few days, which lets them expand and hold more liquid, for instance.

Because we don’t really know how big any particular newborn’s stomach is, the best approach to feeding frequency comes from watching the baby’s behavior. Babies should be fed on demand, the American Academy of Pediatrics recommends. And babies “demand” a meal — either breastmilk or formula — by rooting around with their heads or starting to suck on something (their hands, their parents’ arms, whatever’s within reach). Crying is often one of the last signals they send. Breastfeeding sessions should be attempted eight to 12 times in a 24-hour period.

We’re definitely in round-the-clock territory in my house. During a recent back-to-back nursing session, I had a vague memory of a study on the !Kung hunter-gatherers of Namibia and Botswana. Reading the study again gave me a whole new appreciation for these women. During the daytime (when researchers were observing), the women’s babies, who ranged in age from 12 weeks to more than 2 1/2 years, nursed an average of four times an hour, with about 13 minutes between nursing bouts. Those astonishing numbers put my own nursing marathons into perspective, and reminded me that the struggle to keep up with growing babies’ appetites is universal.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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