New parents marvel at their teeny-tiny babies—those itty-bitty fingers and toes! But for a certain set of extra-small newborns, their size and weight can warrant more concern than awe. The good news is that the vast majority of low birth weight babies grow up to be healthy children and thriving adults. That said, there are some important things to know about potential health complications a low birth weight baby may face in the short- and long-term. So what is considered low birth weight and what are the potential implications? Read on for the lowdown.

In this article:
What is a low birth weight baby
What causes low birth weight?
What low birth weight might mean for baby
Caring for a low birth weight baby

What Is a Low Birth Weight Baby?

The average birth weight is around 8 pounds, per Stanford Medicine Children’s Health. That said, there’s a range of what’s considered normal, and plenty of healthy babies are born weighing more or less than that.

According to the World Health Organization (WHO), roughly 15 to 20 percent of babies born around the world are classified as low birth weight, or weighing less than 5.5 pounds. Within this category, babies are further categorized based on their exact weight. Here’s a quick breakdown:

Moderately low birth weight: Between 3.3 to 5.5 pounds
Very low birth weight: Between 2.2 and 3.3 pounds
Extremely low birth weight: Under 2.2 pounds

It’s also important to note that low birth weight is different than “small for gestational age,” which refers to newborns who weigh less than the 10th percentile for the gestational age at which they’re born. There’s also a distinction between prematurity (being born early) and low birth weight—although many preemie babies have a low birth weight.

What Causes Low Birth Weight?

According to the American College of Obstetricians and Gynecologists (ACOG), there are two primary causes of low birth weight in babies: Intrauterine growth restriction (IUGR) and preterm birth.

IUGR is a condition in which baby is born weighing less than 90 percent of the average weight for their gestational age. According to ACOG, this can occur for a number of reasons related to either a mom’s health, baby’s health or abnormalities with the placenta or umbilical cord, according to ACOG.

A preterm birth is one that happens before 37 weeks gestation; this can be a result of early labor or necessary medical intervention. A baby born early may face medical complications, such as underdeveloped lungs and organs, breathing problems, hearing and vision difficulties, cognitive challenges and even long-term learning issues.

Joanna Parga-Belinkie, MD, IBCLC, an assistant professor of pediatrics and neonatologist at the Children's Hospital of Philadelphia, adds that low birth weight can also be linked to infections, having multiples, insufficient weight gain in pregnancy, age, chronic illness and smoking or drinking in pregnancy. In other words, there are a large number of causes for low birth weight, and it can be difficult to pinpoint the issue.

What’s more, genetics or a parent’s smaller stature could also have something to do with it, says Layan Alrahmani, MD, assistant professor at Loyola University Medical Center in Chicago.

What Low Birth Weight Might Mean for Baby

Research suggests that birth weight can be an important predictor of baby’s health. Still, depending on size, weight and gestational age at birth, there’s a broad range of potential complications and outcomes.

In the short-term, your medical team will monitor your newborn for immediate health issues. These may include low oxygen levels, trouble with temperature regulation, feeding difficulties, infection, breathing problems, bleeding and digestive issues, according to Standford Medicine Children’s Health. Many hospitals in the US require an automatic stay in the Neonatal Intensive Care Unit (NICU) if baby is born under a specific weight, so look into your hospital’s individual policy.

Implications for long-term health after low birth weight

On the spectrum of low birth weight, it’s the youngest, lowest weight babies that face the biggest risks. “It’s very different talking about a 28-week baby versus a 37-week baby weighing the same,” notes Alrahmani.

It’s true that there are statistically higher rates of physical, cognitive and developmental impairments among low birth weight babies. That said, research suggests that the vast majority of low birth weight babies have healthy outcomes. Still, because of higher rates of diseases, such as heart disease, cancer, chronic respiratory disease, obesity and diabetes later in life, the WHO says it’s important to keep up with preventative care throughout childhood into adulthood.

Caring for a Low Birth Weight Baby

Delivering a low birth weight baby may dredge up feelings of helplessness. Fortunately, there are steps you can take to prioritize your little one’s care.

Practice skin-to-skin care. Kangaroo care—or skin-to-skin contact—has been proven to have incredible benefits for baby. In fact, a study out of Harvard found a lower mortality rate among low birth weight babies whose moms practiced skin-to-skin care. What’s more, these newborns had lower rates of infection and higher oxygen levels. Of course, if your little one is admitted into the NICU, you might need to wait until you’re able to hold them to your chest. But once you get the go ahead, be sure to prioritize some quality skin-to-skin cuddle time.
Stay on top of pediatrician appointments. Baby’s doctor may want to see your low birth weight baby more often in the first year of life; this presents the perfect opportunity to ask all your questions about their health, nutrition and growth. While you’re at it, keep a summary of your child’s medical records, so you can have them ready if and when necessary.
Pay attention to baby’s growth and development. Keeping tabs on the Centers for Disease Control (CDC)’s early milestones can help you assess baby’s progress and flag any concerns with their pediatrician in a timely manner.
Look into your state’s early intervention programs. “While general pediatricians will perform developmental screenings, early intervention can screen for more things and also provide services like physical, occupational and speech therapy if needed,” says Parga-Belinkie, adding that developmental services may be available to your infant as they grow at no cost to you. Talk to your pediatrician if you think your child might benefit from additional help.
Keep baby’s sleep space safe. Low birth weight babies are at increased risk of Sudden Infant Death Syndrome (SIDS), and you’ll want to be vigilant about following safe sleep guidelines. As recommended by the American Academy of Pediatrics (AAP), place your little one on their back on a firm, flat surface; remove all soft bedding and additional items from the crib; and keep your little one in your bedroom (in a separate sleep space) until at least the six-month mark.

Medical treatment and monitoring

If you find out you’re at risk for having a low birth weight baby, it doesn’t necessarily mean they’ll have a long stay in the NICU, says Parga-Belinkie. But if baby is admitted to the NICU, you can rest assured that they’ll be closely monitored—a good thing just in case they face complications. Typically, low birth weight NICU babies are placed in temperature-controlled beds, as they may have trouble regulating their body temperature.

While premature and low birth weight babies do need to be carefully watched for potential complications, Parga-Belinkie says they’ll typically be discharged once they’re deemed stable and hit the 4-pound mark (the minimum weight for most infant car seats).

Can You Prevent Low Birth Weight?

Although genetic testing for you and baby during pregnancy can help you detect a heightened risk of low birth weight, it’s impossible to determine exactly why it happens. So while attending all regular prenatal visits, managing chronic conditions, taking prenatal vitamins, eating healthy and exercising are all important proactive steps for a healthy pregnancy, they won’t necessarily prevent baby from being born at a low birth weight or facing related complications, says Parga-Belinkie. That said, there are some things you can stop doing to reduce the risk of having a low birth weight baby: Abstain from alcohol, tobacco and illicit drug use during pregnancy.

It’s nerve wracking to learn that baby’s birth weight is low; depending on your newborn’s size, it may feel downright scary. The important thing to keep in mind is that, while there are risks and possible complications, most low birth weight babies grow up healthy, happy and strong. In the meantime, don’t hesitate to reach out to your pediatrician with any questions or concerns.

About the experts:

Layan Alrahmani, MD, FACOG, is an assistant professor at Loyola University Medical Center in Chicago and the chair of the communications committee for the Society of Maternal Fetal Medicine. She earned her medical degree from the University of Jordan Faculty of Medicine.

Joanna Parga-Belinkie, MD, IBCLC, is an assistant professor of pediatrics and attending neonatologist at the Children's Hospital of Philadelphia, director of the newborn nursery at the Hospital of the University of Pennsylvania. She’s also a certified lactation consultant. She earned her medical degree from the Mount Sinai School of Medicine in New York.

Plus, more from The Bump:

What Is Considered a Full-Term Pregnancy—and Why Does It Matter?

What You Need to Know About Baby Growth Spurts

When Can Newborns Go Outside

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