At birth, most babies weigh 6 to 8 pounds. Low birth weight refers to infants who weigh less than 5.5 pounds at birth. Most normal babies weigh 5.5 pounds by 37 weeks of gestation. Intrauterine growth restriction refers to delayed growth within the uterus, which then leads to low birth weight. Some babies are just small and happen to weigh less than 5.5 pounds at birth, just like some adults are smaller than others. Though this is considered low birth weight, in these cases, it is not abnormal or a cause for concern.
Low birth weight may be suspected before delivery if the size of the mother's uterus is small, or if ultrasound shows a small fetus. The fetus may appear small overall, or have a normal-size head for gestational age but an unusually small body. Although the overall size of the baby is small, the organ systems are appropriately mature for gestational age. If the mother is small, it may be normal for her to have a small baby.
Several factors can cause delayed growth of a fetus. Babies with congenital anomalies or chromosomal abnormalities often have low birth weights. Sometimes problems with the placenta can keep enough oxygen and nutrients from getting to the fetus. Infections during pregnancy that affect the fetus may affect the baby's birth weight. These include:
Risk factors in the mother that may contribute to low birth weight include:
Low birth weight is more common in first-time pregnancies and in pregnant women under age of 17 and over age 35.
Prenatal care is important because there are no symptoms of having a low-birth-weight baby. The most common symptom is simply feeling that the baby is not as big as it should be. During each prenatal visit, your health care provider will carefully measure your abdomen to check in the baby's size. If the measurements do not increase as they should you will likely have an ultrasound. The ultrasound can measure your baby's growth and help determine if there is intrauterine growth restriction.
Low birth rate is also determined when your newborn is examined after delivery. If your baby's weight and length fall below the 10th percentile for their age, then they are considered to be low birth weight.
While there is no specific treatment, it's very important to eat a well-balanced, nutritious diet for you and your baby, both during pregnancy and after birth. Sometimes, risk factors for low birth-weight can be identified early on in the pregnancy. Making behavioral changes and treating any chronic conditions can help reduce these risk factors. Most low birth-weight infants will catch up with the growth of other babies their age somewhere between the 18th and 24th month.
Certain complications may occur if your infant has intrauterine growth restriction. A lack of oxygen during birth, called birth asphyxia, may occur if the growth restriction is caused by problems with the placenta. Meconium aspiration (aspiration of amniotic fluid contaminated with the baby's first stool) may occur due to stress during delivery. There may also be low blood glucose levels during the first hours or days of life.
It's hard for a mother to tell if she's carrying a smaller-than-normal fetus, especially if it's your first pregnancy. But you should, call your provider if you are pregnant and your baby seems very small. You should also call if your infant or child does not seem to be growing or developing at a standard rate.
There are a few things you can do to increase your chances of giving birth to a baby with healthy weight. Proper nutrition, plenty of rest, and avoiding cigarettes, drugs, and alcohol will help ensure you have a healthy child.
Q: What's the difference between low birth weight and premature birth?
A: Low birth weight is often confused with premature birth. Low birth weight refers to a baby's weight at birth. Premature birth refers to a birth occurring before 37 weeks. Measurements of low birth weight have been expanded to include very low birth weight for infants weighing less than 3.3 pounds (1,500 grams) and extremely low birth weight for infants weighing less than 2.2 pounds (1000 grams).
Q: If my first child had a low birth weight, should I be concerned about my next pregnancy?
A: If you had a low birth-weight baby in a previous pregnancy, your risk of having another is moderately higher. The risk depends on the reasons for the prior low birth-weight pregnancy. Be sure to receive good prenatal care and discuss your concerns and your medical history with your provider.
Reviewed By: John D. Jacobson, MD, Professor of Obstetrics and Gynecology, Loma Linda University School of Medicine, Loma Linda Center for Fertility, Loma Linda, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.