Low birth weight: what it means when your baby weighs under 2.5 kg

Low birth weight means your baby weighed less than 2,500 grams, about 5.5 pounds, at birth. That is the WHO definition. It happens when a baby is born early, grows too slowly in the womb, or both. Being small is not the same as being unwell: Cleveland Clinic says some low birth weight babies are healthy, and most catch up in growth when there are no other complications.

Why some babies are born small

The WHO names two causes. A baby born before 37 weeks, a premature baby, has had less time to put on weight, and Cleveland Clinic notes that a baby gains much of its weight in the last few weeks of pregnancy. Other babies arrive at term but did not grow as expected in the womb, which doctors call intrauterine growth restriction (IUGR). Some are simply small because their parents are small.

Cleveland Clinic lists things that raise the chance of a small baby: twins or more, infection or chronic illness in pregnancy, smoking or alcohol, not gaining enough weight, and being 17 or younger or 35 or older. In Singapore, KKH links anaemia in pregnancy to low birth weight, and SingHealth's HealthXchange says gestational hypertension can lead to very low-weight babies.

The care a small baby gets after birth

Small babies have little body fat, so Cleveland Clinic says they can struggle to stay warm, feed, gain weight and fight off infection. Your baby is weighed soon after birth and the weight is compared with how many weeks pregnant you were. Care depends on both. It can mean a stay in the neonatal intensive care unit (NICU), a temperature-controlled bed, and feeds by tube or drip until your baby can feed by mouth.

In Singapore, KKH's dietitians work with low birth weight premature babies from the NICU through to after discharge, including advice on milk feeds and additives for babies who still need to catch up when they go home. KKH's Early Bird Baby Club is a support group for parents of premature babies born under 1,500 g.

Tracking catch-up growth at home

Cleveland Clinic says babies born small usually catch up in physical growth as long as there are no other complications, and that keeping every well-child check matters. NUH schedules well-baby visits at 2 weeks, 6 weeks to 2 months, and 4, 6, 12 and 18 months, and checks growth and development at each. Bring your child's health booklet so the readings stay in one place.

Between visits, our baby growth percentile calculator plots your baby's weight on the WHO charts, so you can see the trend rather than one number. If your baby was born early, ask your paediatrician whether to read the chart by corrected age.

Does low birth weight matter later?

It can. The WHO links low birth weight to slower growth and cognitive development and to long-term conditions later in life. Cleveland Clinic names developmental delays and, in adult life, high blood pressure, heart disease and diabetes. Cleveland Clinic says routine well-child checks are where these are looked for, which is one more reason to keep every appointment.

Frequently asked questions

What is low birth weight in kg?

Under 2.5 kg. Cleveland Clinic gives the full bands: low birth weight is under 2.5 kg, very low birth weight under 1.5 kg, and extremely low birth weight under 1 kg.

Is a 2.4 kg baby too small?

By the WHO line it counts as low birth weight, but that alone does not mean something is wrong. Cleveland Clinic says some low birth weight babies are healthy, and doctors judge your baby's weight against how many weeks pregnant you were.

Will my baby catch up?

Most do. Cleveland Clinic says babies with low weight at birth usually catch up in physical growth when there are no other complications. At KKH, dietitians follow up babies who still need to catch up after discharge.

What is the meaning of low birth weight versus premature?

Premature is about timing: born before 37 weeks. Low birth weight is about size: under 2,500 g. The WHO says low birth weight comes from being born early, growing slowly in the womb, or both, so a baby born at term can still be small.

Related terms

• Preterm (Premature) Birth: A baby born alive before 37 completed weeks of pregnancy. In Singapore, about 8.5% of babies arrive this early.
• Gestational Age: How far along your pregnancy is, in weeks and days, counted from the first day of your last period.
• Growth Percentile: How your baby's weight, length or head size compares with children of the same age, as plotted on a growth chart.
• Health Booklet: The Child Health Booklet every Singapore baby gets at delivery. It records growth, milestones and every vaccination.
• Neonate (Newborn): The medical word for a newborn baby in the first 28 days of life.
• Anaemia: Too few healthy red blood cells or too little haemoglobin, most often from low iron during pregnancy.

See every term in the glossary