Why Most Newborns Lose Weight After Birth

Part of the reason why the third trimester of pregnancy is so uncomfortable is due to the increased weight of the baby as they grow rapidly in utero. The American Pregnancy Association states that by the 32nd week of pregnancy, a baby can weigh over 4 pounds and measure up to 17 inches long. They begin to gain fat stores until they're born. In fact, the weight gain doesn't slow down and by 36 weeks, they may have packed on an additional 2 pounds. Once the baby reaches full term, at about 38 weeks, it'll measure an average of 20 inches and weigh anywhere from around 7 to 10 pounds. Cue the back and pelvic pain.

If you're wondering how big your baby will be at birth, there are several factors that contribute to its debut weight. First and foremost, the gestational age at which they are born will be the main factor in their birth weight, as per Nemours KidsHealth. Other influences include genetics, sex, as boys tend to be larger than girls, certain health conditions during pregnancy, such as diabetes and obesity, the existence of any birth defects, and the diet of the gestating individual.

Regardless of your baby's weight at birth, all newborns ultimately end up losing a bit of weight once they make their way earthside.

A newborn will ultimately lose anywhere from 7% to 10% of their birth weight in the first days after being born, in part because of fluid loss (per Nemours KidsHealth). In fact, there are various changes that must take place when a baby is no longer in utero, such as the need to regulate their body temperature, use their lungs to breathe, and establish balanced fluid levels, explains Medline Plus. All of these tasks require excess energy in the form of calories from food. Luckily, once proper feeding patterns are established, a baby will regain the weight lost within a couple of weeks (via Nemours KidsHealth).

Although a baby's growth can fluctuate, the Mayo Clinic states that most will gain around an ounce per day for the first four months of life. From four to six months, the weight gain changes to about 0.70 ounces and then decreases again at six months to 0.35 ounces a day. As for height, most babies gain about an inch a month until their first half-birthday, then half an inch a month until their first birthday. It is most important that a baby follow their unique growth curve, regardless of how much weight they gain within the first year.

In some cases, a baby doesn't follow the average guidelines for birth weight and is born either weighing too little or too much. Low birth weight is defined as anything lower than 5 pounds, 8 ounces, explains March of Dimes. While there is no guarantee that a baby weighing less than this will be unhealthy, there are some risks that go along with a low birth weight. These include difficulties feeding and breathing, an inability to fight off infections, and jaundice in the short term. They may also develop health issues later in life, such as diabetes and high blood pressure.

On the flip side, a baby may be born with a high birth weight or come in weighing more than 8 pounds, 13 ounces, as per the Mayo Clinic. That being said, the risks that come with being born heavier than average increase after 9 pounds, 15 ounces. Similar to being born with a low weight, high-weight babies may experience some complications as they grow, including low blood sugar levels, obesity, or metabolic syndrome, a condition that can lead to heart disease and diabetes.

Therefore, proper prenatal care can go a long way in mitigating the risks of a baby being born with a low or high birth weight, explains Cedars Sinai. Additionally, once born, they should be monitored by a medical professional for any potential health complications to ensure healthy development.