Most cases are mild and go away on their own, but some infants need to stay in the hospital for treatment with UV lights.

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Beyoncé and Jay Z’s twins have arrived, People reported over the weekend, and on Monday, the new dad was spotted at Ronald Reagan UCLA Medical Center in Los Angeles.

Aside from an Instagram post from Beyoncé’s father announcing the twins’ births, there’s been no official word from the Carter camp. But TMZ reported yesterday that the twins were being kept at the hospital “under the lights,” which could mean that they’re being treated for jaundice—a condition that causes yellowing of the skin and eyes in newborn babies.

Health has not independently confirmed that story. But regardless, jaundice is extremely common in infants—and not usually cause for serious concerns, says DeeAnne Jackson, MD, medical director for the University of Alabama at Birmingham Newborn Nursery. (Dr. Jackson has not treated Beyoncé or her children.)

Because the condition is suddenly in the news, we asked Dr. Jackson to give a rundown on what it is and how it’s treated. Here’s what’s important to know.

Jaundice is usually related to the baby’s liver

Jaundice, which occurs in up to 60% of newborns, occurs when a baby’s blood contains a buildup of bilirubin—a yellow-colored pigment that’s formed when red blood cells break down. Normally, the liver filters these pigments out of the bloodstream and gets rid of them with the body’s waste, but sometimes the baby’s liver “isn’t quite working at full speed,” says Dr. Jackson.

Rarely, jaundice can be caused by other problems, such as internal bleeding, an infection, or an abnormality in the baby’s red blood cells. Some breastfeeding babies also develop jaundice after their first week of life.

Yellow-tinted skin or eyes is a sign

Because bilirubin has a yellow coloring, it can cause babies’ skin—and the white of their eyes—to appear yellowish, as well. Sometimes this is obvious just by looking at an infant; other times, the skin may only look yellow after a parent or doctor presses gently on a baby’s nose or forehead. In babies with darker skin, the yellow tint may not be noticeable.

Babies are screened for jaundice while they’re in the hospital, and should continue to be monitored for the first week of their life, even after they go home. (Bilirubin levels usually peak between the third and seventh day of life.) Babies should also have their bilirubin levels tested if they have don’t have a normal amount of wet or dirty diapers, if they’re not eating well or not gaining weight, or if they have a shrill, high-pitched cry and can’t be comforted.

It’s more common, and can be more serious, in premature babies

Babies born before their due date—even just a week or two early—are more likely to develop jaundice than those born later, says Dr. Jackson, since their livers may not yet be mature enough to filter out the bilirubin in their blood. Babies born pre-term, before 37 weeks, have an even greater chance.

While we don’t know whether Beyonce’s twins were born prematurely, moms of twins are at higher risk of delivering pre-term. Even when all goes according to plan, twins are usually born a few weeks earlier than singletons.

Most cases don’t require treatment …

Usually when babies have elevated bilirubin levels, they don’t need any special treatment and they get better on their own within a few weeks.

Because humans get rid of bilirubin through bodily waste, it’s important to make sure a baby with jaundice (and all babies, for that matter) is producing a normal amount of stool. If a baby’s skin or eyes still has a yellow tint after three weeks—or if she’s not eating well or you’re having trouble breastfeeding—talk to your doctor.

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… But some babies need phototherapy

“Jaundice is normal, but there are rare cases where the bilirubin levels can get so high that it can become toxic,” says Dr. Jackson. Untreated, this can cause brain damage, which is why it’s so important for doctors to catch it early.

When levels do get too high, babies are treated with special UV lights that penetrate through the skin and help break down bilirubin in the blood. “Sometimes we use lights that are placed right over the infant’s crib, and sometimes they lie on a little bed with lights under the mattress,” says Dr. Jackson. “There are even some light blankets that are flexible and wrap around the baby to expose their skin.” (Babies should never be treated with direct sunlight, she warns, which can cause sunburn.)

During phototherapy, babies only wear a diaper and soft goggles or patches to protect their eyes. These treatments are usually only needed for a day or two, although premature babies may need to stay in the hospital for longer because of other complications. Rarely, a baby with very high bilirubin levels may require a blood transfusion.

It’s one of the most important reasons for infant check-ups

“We used to discharge babies and say, ‘Come back in two weeks,’” says Dr. Jackson. “But now, we want babies who are sent home within 48 hours of birth to be seen again within three days. One of the main reasons for that is so they can be evaluated for jaundice a few days after birth.”

Most of the time, parents shouldn’t stress about a jaundice diagnosis, says Dr. Jackson—even if their child does require phototherapy. “We tend to be very conservative and treat pretty early,” she says, “so that babies don’t get anywhere close to the point where they’ll be at risk for toxicity or need a more aggressive treatment.”