preterm labor, labor that occurs earlier in pregnancy than normal, either before the fetus has reached a weight of 2000 to 2500 g or before the 37th or 38th week of gestation. No single measure of fetal weight or gestational age is used universally to designate preterm birth; local or institutional policy dictates which of several standards is applied. Prematurity is a concomitant of 75% of births that result in neonatal mortality. It may occur spontaneously, or it may be iatrogenic. The incidence of preterm labor increases in inverse proportion to maternal age, weight, and socioeconomic status. Incidence is higher for African-American women, women who have not had adequate prenatal care or have an abnormal obstetric history, and women who smoke or whose diet is deficient in protein or calories. Predisposing conditions include maternal infection, low weight gain, uterine bleeding, multiple gestation, polyhydramnios, uterine abnormalities, incompetent cervix, premature rupture of membranes, and intrauterine fetal growth delay. The cause of preterm labor is poorly understood; in some cases there may be several contributing causes. In some pregnancies preterm labor may be homeostatic, resulting in the best possible outcome under the particular abnormal conditions. If preterm labor itself constitutes a threat to the fetus, the outcome of pregnancy may be improved if labor can be inhibited. Determining accurately which pregnancies are likely to benefit from the inhibition of labor and which are not is difficult. Medications used to stop labor are not always effective. Misdiagnosis of gestational age and fetal condition may lead to induction of labor that is inadvertently premature; preterm babies whose birth has been brought about inappropriately early account for 15% of admissions to newborn intensive care nurseries. Also called premature labor. See also small for gestational age (SGA) infant.
