The most common “cause” of postterm pregnancy is inaccurate estimation of gestational age (dating.) Inaccurate dating ismore likely in women with irregular menses and, thus, in-consistent ovulation; women who seek prenatal care later in pregnancy; women with delayed ovulation (for exam-ple, women who have recently discontinued oral contra-ceptives); and women who inaccurately recall their LMP. Inaccurate dating that leads to the erroneous classification of a pregnancy as postterm has important sequelae. These pregnancies are labeled “high-risk.” Costly increased eval-uations are undertaken and the likelihood of intervention increases, specifically, delivery by induction of labor or by cesarean section, which are potentially associated with increased maternal and fetal morbidity. Other less common causes of postterm pregnancy are listed in Table 23.1.
Whatever the cause, there is a tendency for recurrence of postterm pregnancy. Approximately 50% of patients whohave one postterm pregnancy will experience prolonged pregnancy with the next gestation. Other important risk factors includematernal obesity, nulliparity, and postterm delivery of the mother. There also appears to be a genetic influence, based on twin studies.