Prolonged pregnancy or postdate pregnancy has been defined as one continuing beyond 280days (40weeks) from a known time of ovulation or conception or beyond 42weeks (294days) from the first day of her last menstrual period.
The incidence of post term pregnancies varies significantly across the globe. A figure of 5.5 – 8.5% was quoted in America, 0.4 – 8.1% in Europe and 2 – 19% in Nigeria.
The aetiology of prolonged pregnancy remains unclear, factors believed to play a role include wrong dates, previous history of prolonged pregnancy, hereditary factors, fetal anencephaly, placental sulphatase deficiency, age and parity, fetal adrenocortical insufficiency.A significant number of pregnancies classified as prolonged are as a result of error in dating. Methods of assessing gestation age comprises: calculating from the date of the last menstrual period, assessment of uterine size and measurement of crown-rump length by a first trimester ultrasound.
To certify a pregnancy as postdated, the client must be sure of her last menstrual period. A properly kept menstrual calendar will ensure accuracy in date. Antenatal fundal height measurement must correspond with the booking date and above all an early first trimester ultrasound scan must be compatible with menstrual date. In cases when there is doubt about the gestational age, the earliest scan must be used preferably scan done before 20 weeks gestation.
Delivery at earlier gestation may prevent increase in perinatal mortality and morbidity associated with prolonged pregnancy. If a woman opts to wait after the required due time, she should be informed about the risk involved in the continuation of the pregnancy beyond 41weeks viz a viz increased fetal and neonatal morbidity and mortality. Perinatal morbidity is increased due to placental insufficiency, increased risk of fetal hypoxia, macrosomia, meconium aspiration syndrome and their sequelae. The use of routine ultrasound for dating in the first trimester has decreased the overall rate of Post term pregnancy. In the light of current evidence, earlier intervention with induction of labour at 41weeks appears appropriate management.