Fetal growth and timing of parturition in humans.

Animal studies indicate that either the fetus or the intrauterine environment, both of which set the pattern for fetal growth, may affect the timing of parturition. The authors examined the association between fetal growth and timing of spontaneous onset of labor in humans among low-risk white US women with singleton pregnancies (1987-1991). They restricted the data to pregnancies which had a reliable date of the last menstrual period, normal fetal growth in the first half of pregnancy, and no history of or current pregnancy complications that might have impaired fetal growth (n = 3,360). Subjects received ultrasound examinations at 15-22 and 31-35 weeks' gestation. Fetal growth was adjusted for parity, fetal sex, and maternal prepregnancy weight and height. Results showed that slower or faster fetal growth in the second half of pregnancy resulted in substantially lower or higher birth weight, respectively. However, fetal growth in the second half of pregnancy, even at extremes (2 standard deviations below or above the mean), did not have a meaningful impact on the timing of parturition; neither did fetal growth acceleration or deceleration in late pregnancy. Thus, in low-risk pregnancies where fetal growth is normal in early gestation, fetal growth in the second half of pregnancy does not affect the timing of normal parturition.