The primary cause is premature birth, being born before 37 weeks gestation; a baby born early has less time in the mother’s uterus to grow and gain weight, and much of a fetus’s weight is gained during the latter part of the mother’s pregnancy.
Another cause of low birthweight is intrauterine growth restriction. This occurs when a baby does not grow well in utero because of problems with the placenta, the mother’s health or birth defects. Babies with Intrauterine growth restriction (IUGR) may be born early or full-term; premature babies with IUGR may be very small and physically immature, and full-term babies with IUGR may be physically mature but weak.
Any baby born prematurely is more likely to be small. However, there are other factors that can also contribute to the risk of low birthweight. These include:
Race – African-American babies are twice as likely as Caucasian babies to have low birthweight.
Mother’s age – Teen mothers (especially those younger than 15) have a much higher risk of having a baby with low birthweight.
Multiple birth – Multiple birth babies are at increased risk of low birthweight because they often are premature.
Mother’s health – Babies of mothers who are exposed to illicit drugs, alcohol and cigarettes are more likely to have low birthweight. Mothers of lower socioeconomic status are also more likely to have poorer pregnancy nutrition, inadequate prenatal care, and pregnancy complications — all factors that can contribute to low birthweight.
Nearly all low birthweight babies need specialized care in the Neonatal Intensive Care Unit (NICU) until they gain weight and are well enough to go home. Fortunately, there is a 95 percent chance of survival for babies weighing between 3 pounds, 5 ounces and 5 pounds, 8 ounces.