What is a Low Birth Weight Baby?
Updated September 03, 2013.
Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
Low birth weight (LBW) is the medical classification for a baby who weighs less than 2500 grams—or 5 lbs 5 oz—at birth. Any baby who weighs less than 2500 grams at birth is classified as LBW, whether the baby was born premature or at term.
A low birth weight baby will fall into one of 3 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.
What Causes Low Birth Weight?
Babies are born small for 2 main reasons: either 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, including:- Prematurity: A premature baby is any baby born before 37 weeks of pregnancy. Because they haven't had as much time to grow, many premature babies are small at birth.
- Preeclampsia, or other problems with the pregnancy:Preeclampsia is a condition of pregnancy that can cause high blood pressure and other symptoms in the mother. The baby may not be nourished well, and may grow more slowly than expected. Other problems with a pregnancy can cause a baby to be born small, either because the baby was premature, or because it didn't get enough nutrients during pregnancy.
- Smoking or substance abuse: Smoking cigarettes, drinking alcohol, and using illegal drugs are all bad for your baby in many ways. Substance abuse during pregnancy can cause your baby to be smaller than normal, which can cause a number of problems—both in the newborn period and later in life.
- Multiple birth (twins or more): Twins, triplets, and higher-order multiples are often smaller at birth than other babies born at the same gestational age. They also have a higher chance of being born premature, which is a separate risk factor for low birth weight.
- Poor nutrition in the mom: It's important to eat right when you're pregnant. If you don't eat the right foods or don't eat enough, your poor pregnancy nutrition may not allow your baby to grow well.
- Infection in the mom or baby prior to birth: Infections during pregnancy can cause a baby to be born premature, or to have restricted growth during pregnancy. Infections in the uterus can cause also preterm labor and birth. Other infections can cause low birth weight when they affect the baby, including cytomegalovirus (CMV), toxoplasmosis, chickenpox, and rubella.
Can Low Birth Weight Harm My 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.
Other low-birth weight babies are not so lucky, and may have a number of problems related to their birth weights.
- Problems related to prematurity: Babies who are small because they were born early are at risk for health problems related to prematurity. These complications of prematurity can include problems with the 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.
March of Dimes. Medical Resources: Low Birthweight. Retrieved from http://www.marchofdimes.com/professionals/medicalresources_lowbirthweight.html
Lucile Packard's Children's Hospital at Stanford. "Very Low Birthweight" Retrieved from http://www.lpch.org/DiseaseHealthInfo/HealthLibrary/hrnewborn/vlbw.html
UCSF Children's Hospital. Intensive Care Nursery House Staff Manual. "Very Low and Extremely Low Birthweight Infants" Retrieved from http://www.ucsfbenioffchildrens.org/pdf/manuals/20_VLBW_ELBW.pdf