Low Birth Weight
A baby born weighing less than about 5.5 pounds
Quick Facts
- Type: Newborn condition
- Definition: Birth weight under about 2,500 grams (5.5 lb)
- Main causes: Prematurity, restricted growth in the womb
- Care setting: May need extra newborn or NICU care
Overview
Low birth weight describes a baby born weighing less than about 5.5 pounds, or 2,500 grams. It usually happens for one of two reasons: the baby was born early (premature) and had less time to grow, or the baby did not grow as expected during pregnancy, a situation called intrauterine growth restriction.
Babies with low birth weight may need extra medical attention because being small can affect how well they breathe, feed, and stay warm. Many small babies catch up and thrive with appropriate care, while very small or very early babies may face more challenges and need specialized newborn care.
Features
A low-birth-weight baby is mainly identified by weight at birth, but may also have:
- A small, thin appearance with little body fat
- A head that looks large relative to the body
- Difficulty staying warm
- Trouble feeding or a weak suck
- Lower energy and more sleepiness
- Greater vulnerability to infection
The degree of these features depends on how small the baby is and whether the low weight is from prematurity, growth restriction, or both.
Causes
The two leading reasons a baby has a low birth weight are:
- Premature birth: Babies born before 37 weeks have had less time to gain weight.
- Intrauterine growth restriction: The baby grows more slowly than expected in the womb, often due to problems with the placenta or blood flow.
Contributing factors include high blood pressure or preeclampsia, infections, carrying twins or more, problems with the placenta, and the pregnant person's health and nutrition. Smoking, alcohol, and drug use during pregnancy also raise the risk.
Risk Factors
- Premature delivery
- High blood pressure or preeclampsia during pregnancy
- Carrying twins, triplets, or more
- Smoking, alcohol, or drug use in pregnancy
- Poor nutrition or low weight gain during pregnancy
- Infections during pregnancy
- Problems with the placenta
Diagnosis
Low birth weight is diagnosed by weighing the baby at birth. During pregnancy, providers may suspect poor growth and monitor it with:
- Ultrasound to estimate the baby's size and growth.
- Measurement of the uterus at prenatal visits.
- Blood flow studies to check the placenta when growth restriction is suspected.
After birth, the baby is assessed for breathing, temperature, feeding, and blood sugar.
Care and Treatment
Care is tailored to the baby's size and needs.
- Warmth: Skin-to-skin contact and, when needed, an incubator to maintain temperature.
- Feeding support: Help with breastfeeding, expressed milk, or tube feeding for babies who cannot yet feed well.
- Monitoring of breathing, blood sugar, and growth.
- NICU care for very small or premature babies, which may include breathing and nutritional support.
- Treatment of any underlying problems, such as infection.
Many low-birth-weight babies grow and catch up over time with good nutrition and follow-up.
Prevention
- Attend regular prenatal care so growth can be monitored
- Eat a balanced diet and aim for healthy weight gain in pregnancy
- Avoid smoking, alcohol, and recreational drugs
- Manage chronic conditions such as high blood pressure and diabetes
- Treat infections promptly during pregnancy
When to See a Doctor
During pregnancy, contact your provider if you notice decreased movement of the baby or have concerns about growth. After birth, seek care if a small baby:
- Is feeding poorly or not waking to feed
- Is unusually sleepy, floppy, or hard to rouse
- Has a fever, difficulty breathing, or a bluish color
- Is not gaining weight as expected
Seek emergency care for any breathing difficulty or a baby who is hard to wake.
Frequently Asked Questions
What weight is considered low birth weight?
Low birth weight means a baby is born weighing less than about 5.5 pounds, or 2,500 grams. Babies under about 3.3 pounds are considered very low birth weight and usually need more intensive newborn care.
What causes a baby to have a low birth weight?
The two main causes are premature birth, where the baby has had less time to grow, and intrauterine growth restriction, where the baby grows slowly in the womb. Factors such as high blood pressure, multiple pregnancy, infections, poor nutrition, and smoking can contribute.
Will a low-birth-weight baby be healthy?
Many low-birth-weight babies grow and catch up well with good nutrition and follow-up care. Very small or very premature babies may face more challenges and need specialized care, but outcomes have improved greatly with modern newborn medicine.
Can low birth weight be prevented?
Not always, but the risk can be reduced with regular prenatal care, good nutrition and healthy weight gain, avoiding smoking and substances, and managing chronic conditions and infections during pregnancy.
References
- Centers for Disease Control and Prevention (CDC).
- World Health Organization (WHO). Low birth weight.
- March of Dimes. Low birthweight.
- MedlinePlus, U.S. National Library of Medicine. Premature babies.