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Abstract
Postterm pregnancy is defined as the one which persists for 41 completed weeks or more, calculated from the first day of the last menstrual period (LMP). Most of the cases have no obvious cause for this prolongation.
The incidence of postterm pregnancy averages between 2.5-12% of pregnancies, depending on the method used for assessment. Most published data agree that 80% of births occur between 38 and 42 weeks.
Oligohyramnios is the factor most frequently associated with prolonged pregnancy. Oligohydrmanios is associated with increased rates of perinatal morbidity and mortality. The principal reasons for increased risks to postterm fetuses were the consequence of cord compression associated with oligohydramnios. Decreased amnionic fluid volume commonly develops as pregnancy advances beyond 42 weeks. It is also likely that meconium release into an already reduced amnionic fluid volume causes thick, viscous meconium implicated in meconium aspiration syndrome.
Registry data indicate that the increased risk of stillbirth in post-term pregnancies is partly due to an increased risk of small-for-gestational-age (SGA) fetuses; the latter is in turn partly due to an increased risk of congenital anomalies. When malformations were excluded, the post-term SGA had a higher stillbirth rate than term SGA and post-term appropriate-for-gestational-age fetuses. Similarly, the risk of 5-min Apgar score < 5 was higher among the post-term SGA than the other two groups. As a result of data indicating an elevated risk of adverse perinatal outcomes, antenatal testing is now a standard feature of the clinical management of prolonged gestations. There is convincing scientific evidence that Doppler velocimetry of the umbilical vessels identifies uteroplacental insufficiency in high-risk pregnancies. Furthermore, it has been shown that when umbilical Doppler information is made available to clinicians it improves decision making and ultimately the outcomes in such pregnancies. Not surprisingly, therefore, there was initial interest and optimism in deploying Doppler for the management of prolonged gestation.
Doppler velocimetry of the renal artery, while of limited practical value in the day-to-day management of prolonged pregnancies, has the potential to shed light on the mechanism of fetal deterioration.