Low Birth Weight Baby Risks, Types, and Causes

Premature baby

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Low birth weight (LBW) is the medical classification for a baby who weighs less than 2500 grams—or 5 pounds, 5 ounces—at birth. LBW babies are at higher risk of health complications but not all low birth weight babies will need interventions or special care, particularly those that are close to the 5 pound, 5 ounce weight cut-off.

While it may feel scarier to care for a baby born at low birth weight, there aren't necessarily many differences in the day-to-day care of your newborn. However, family members of a low birth weight baby need to be extra vigilant to ensure their child stays healthy. Learn more about low birth weight baby risks, types, and causes.

What Is a Low Birth Weight Baby?

Babies are weighed soon after birth. If their weight falls below 5 pounds, 5 ounces then they are LBW. Whether your baby was born premature or at term, they can be classified as a low birth weight baby. The impact of being low birth weight, including any extra medical care they may need will vary greatly based on how small they are, how far along they were when they were born, their medical condition, and many other factors.

A low birth weight baby will fall into one of three categories:

  • Low birth weight (LBW): A LBW baby weighs less than 2500 grams, or 5 lbs 5 oz.
  • Very low birth weight (VLBW): A VLBW baby weighs less than 1500 grams, or about 3 lb 9 oz.
  • Extremely low birth weight (ELBW): An ELBW baby weighs less than 1000 grams, or about 2 lb 3 oz.

Causes of Low Birth Weight

Babies are born small for two main reasons: they were born early or they were born on time but didn't grow enough during pregnancy (called intrauterine growth restriction, or IUGR). There are many specific causes of low birth weight.

These include prematurity, preeclampsia, or other problems with the pregnancy, smoking or substance abuse, multiple birth (twins or more), poor pregnancy nutrition, infection in the mom or baby prior to birth, including cytomegalovirus (CMV), toxoplasmosis, chickenpox, and rubella.

Impact on You and Your Baby

Many people think that having a baby that's born on time and just small, or a baby who's just a little early, won't cause the baby any problems. The fact is that most low birth weight babies do just fine, and have few (if any) problems caused by their small sizes. However, there are some exceptions. Here are problems low birth weight babies may experience:

  • Issues with internal organ function: Babies born prematurely may have complications of prematurity that include problems with the function of their brain, heart, lungs, intestines, and more.
  • Problems with blood sugar: Very small babies may have trouble regulating their blood sugar. Late preterm babies sometimes use sugar faster than they can replace it, and can easily develop dangerously low blood sugars.
  • Problems staying warm: Small babies don't have enough fat to keep them warm. If they can't stay warm on their own, they may have to spend time in an incubator.
  • Trouble eating: Smaller babies aren't always strong enough to breastfeed or bottle-feed well, and may need help taking in enough calories to grow.

Watching for Complications

While you cannot control the nature and severity of your baby's weight on their health, you can be watchful for complications. Premature babies are typically monitored more regularly than normal weight babies. Expect to be extra careful if your low weight baby has trouble feeding, keeping warm, or shows signs of infection.

As they grow older, studies show they may be more prone to health conditions, including asthma, vision problems, and fine motor skills and hand-eye coordination.

The bright side? Research from the longest running study of premature infants shows they are exceptionally resilient and may have an increased drive to succeed. What's more, parents who show more concern and advocacy for their well-being in school and social settings turn out with children who become more successful academically, socially, and physically.

2 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Hughes MM, Black RE, Katz J. 2500-g Low Birth Weight Cutoff: History and Implications for Future Research and Policy. Matern Child Health J. 2017;21(2):283-289. doi:10.1007/s10995-016-2131-9

  2. Cutland CL, Lackritz EM, Mallett-Moore T, Bardají A, Chandrasekaran R, Lahariya C, Nisar MI, Tapia MD, Pathirana J, Kochhar S, Muñoz FM; Brighton Collaboration Low Birth Weight Working Group. Low birth weight: Case definition & guidelines for data collection, analysis, and presentation of maternal immunization safety data. Vaccine. 2017;35(48 Pt A):6492-6500. doi:10.1016/j.vaccine.2017.01.049

Additional Reading
  • Sullivan, Mary C.; Zigler, Jim. URI College of Nursing study finds effects of premature birth can reach into adulthood. University of Rhode Island Press Release.

  • Lucile Packard's Children's Hospital at Stanford Online. "Very Low Birthweight."
  • March of Dimes Online. Medical Resources: Low Birthweight.
  • UCSF Children's Hospital. Intensive Care Nursery House Staff Manual. "Very Low and Extremely Low Birthweight Infants."

By Cheryl Bird, RN, BSN
Cheryl Bird, RN, BSN, is a registered nurse in a tertiary level neonatal intensive care unit at Mary Washington Hospital in Fredericksburg, Virginia.