Low Birth Weight (LBW)

How low birth weight happens

A baby's weight at birth depends on how long the pregnancy lasted and how fast the baby grew.

Preterm birth

Babies born before 37 weeks have had less time to grow. Most low birth weight in high-income countries is due to preterm birth. Causes include spontaneous preterm labor, early rupture of membranes, and planned early delivery for problems such as preeclampsia.

Fetal growth restriction

Intrauterine growth restriction means the baby is not reaching its growth potential, usually because the placenta is not delivering enough oxygen and nutrients. These babies are often born at term but small for gestational age (SGA). In low-income countries, growth restriction from maternal undernutrition is a major cause.

Causes and risk factors

Maternal health

• high blood pressure, preeclampsia and gestational hypertension
• diabetes with blood vessel disease, kidney disease, heart or lung disease, autoimmune disease
• low pre-pregnancy weight or poor weight gain in pregnancy
• age under 17 or over 35
• short time between pregnancies (less than 18 months)
• previous preterm or low birth weight baby

Pregnancy factors

• twins, triplets or more (multiple pregnancy)
• placental problems: placental insufficiency, placental abruption, placenta previa
• infections: cytomegalovirus, toxoplasmosis, rubella, syphilis, malaria, and vaginal or urinary infections linked to preterm labor
• fetal chromosomal or genetic conditions and birth defects

Lifestyle and environment

• Smoking: one of the most important preventable causes, roughly doubling the risk.
• alcohol, cocaine, methamphetamine and opioids
• air pollution and lead exposure
• stress, poverty, limited access to prenatal care, and racial disparities linked to structural inequities

Complications

The smaller and earlier the baby, the higher the risk. Early problems include:

• breathing difficulty (respiratory distress syndrome) in preterm babies
• low blood sugar and low body temperature
• feeding problems and slow weight gain
• jaundice
• infections and sepsis
• bleeding in the brain and necrotizing enterocolitis (bowel damage) in very low birth weight babies
• retinopathy of prematurity (eye blood vessel problems)

Longer-term risks include cerebral palsy, developmental delay, learning and attention difficulties, vision and hearing problems, and as adults, higher rates of high blood pressure, type 2 diabetes and heart disease.

Diagnosis in pregnancy and at birth

• During pregnancy: dating ultrasound early in pregnancy, fundal height measurements, growth ultrasounds estimating fetal weight, and Doppler studies of blood flow in the umbilical artery when growth is restricted.
• At birth: the baby is weighed and the weight is plotted against gestational age on a growth chart to classify them as appropriate, small or large for gestational age.

Management

During pregnancy

If growth restriction is found, the team monitors with ultrasound, Doppler studies and fetal heart rate tracing, and times delivery to balance the risks of staying in the womb against being born early. Steroid injections may be given to the mother before an expected early birth to mature the baby's lungs, and magnesium sulfate to protect the baby's brain before very preterm birth.

After birth

Care depends on size and maturity: from routine care with extra feeding support to neonatal intensive care with incubators, breathing support and tube feeding. Kangaroo (skin-to-skin) care is recommended by WHO for stable low birth weight babies. For practical home care, see low birth weight baby care.

Prevention

• Start prenatal care early and attend all visits.
• Stop smoking, vaping and alcohol. Help is available through 1-800-QUIT-NOW in the US.
• Treat high blood pressure and diabetes before and during pregnancy. Low-dose aspirin from 12 weeks lowers preeclampsia risk in women at high risk, but only when prescribed.
• Aim for healthy weight gain and take prenatal vitamins with folic acid.
• Space pregnancies at least 18 months apart.
• Get tested and treated for infections, and keep vaccines up to date.
• Progesterone or a cervical stitch may be offered to some women at high risk of preterm birth.

Outlook

Most babies born near 2,500 g, especially near term, grow and develop normally. Survival and outcomes for very small babies have improved greatly with modern neonatal care. Regular follow-up of growth and development helps catch problems early.

Frequently asked questions

What causes low birth weight?

The two main causes are being born too early (before 37 weeks) and growing too slowly in the womb, often due to placental problems. Risk factors include smoking, high blood pressure and preeclampsia, twins or triplets, infections, poor nutrition, and very young or older maternal age.

What is the WHO definition of low birth weight?

The World Health Organization defines low birth weight as a weight at birth of less than 2,500 grams (5.5 pounds), regardless of how many weeks pregnant the mother was.

Can low birth weight be prevented?

Many cases can. Early prenatal care, stopping smoking and drug use, managing high blood pressure and diabetes, eating well, spacing pregnancies and treating infections all lower the risk. Some causes, such as certain genetic conditions, cannot be prevented.

Does low birth weight affect a child later in life?

Most children born with low birth weight grow up healthy. Those born very small or very early have higher risks of developmental and learning difficulties. As adults, low birth weight is associated with somewhat higher risks of high blood pressure, diabetes and heart disease.

What is the difference between low birth weight and small for gestational age?

Low birth weight is a fixed cutoff of under 2,500 grams. Small for gestational age means smaller than 90% of babies born at the same stage of pregnancy. A preterm baby can be low birth weight but appropriately sized, and a term baby can be SGA without being under 2,500 grams.