Doctors are interested in birth weight for many reasons. Babies who weigh much less than average can risk developing a variety of health problems, including failure to thrive. Because of this, doctors try to ensure that babies weigh at least 2,500 grams (5 pounds, 8 ounces) at delivery, although babies who weigh less than this can still be healthy. Babies continue to gain weight the longer they remain in the uterus, so the simplest way to avoid having an underweight baby is to ensure that the pregnancy lasts at least 37 weeks (which is called a "full term" pregnancy). This explains why doctors recommend bed rest and medication for women who are risk of delivering prematurely (before 37 weeks have elapsed). However, even full term babies can weigh less than 2,500 grams if they grow more slowly than they should.

Doctors are also concerned about babies who weigh much more than average. This condition is called "fetal macrosomia" and is usually defined as a birth weight of more than 4,000 grams (8 pounds, 13 ounces). Macrosomia is particularly important during the delivery process itself, because delivering an overweight baby risks injuring both the mother and the baby. In fact, doctors who feel that a baby is too large will often schedule a Cesarean delivery, to minimize the risk of trauma to the mother and the baby. A potentially better option – which requires accurate birth weight predictions – is to deliver the baby after the pregnancy reaches at least 37 weeks, but before the baby becomes too large.

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Some medical references for further reading:
Gregory KD, Henry OA, Ramicone E, et al.: Maternal and infant complications in high and normal weight infants by method of delivery. Obstetrics & Gynecology, 1998, vol 92, pp 507-513.

Patterson, RM, Prihoda TJ, Gibbs CE, et al.: Analysis of birth weight percentiles as a predictor of perinatal outcome. Obstetrics & Gynecology, 1986, vol 68, pp 459-463.

Williams, RL, Creasy RK, Cunningham GC, et al.: Fetal growth and perinatal viability in California. Obstetrics & Gynecology, 1982, vol 59, pp 624-632.

Wilcox AJ, Skjoerven R: Birth weight and perinatal mortality: The effect of gestational age. American Journal of Public Health, 1992, vol 82, pp 378-382.