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Post-term Pregnancy

A normal term pregnancy has a duration of 38 to 42 weeks. A post-term pregnancy is defined as a pregnancy that extends beyond 42 weeks. The incidence of post-term pregnancy is 3-12% for prolonged pregnancy and past due pregnancy.

Causes of Post-Term Pregnancy.  Incorrect dates account for two-thirds of the pregnancies designated "post-term" pregnancies.

Some pregnancies will last more than 42 weeks and have no underlying abnormality. Post-term pregnancy is rarely associated with low estrogen levels, including anencephaly, fetal adrenal hypoplasia, absence of fetal pituitary, estrogen precursor deficiency, or placental sulfatase deficiency.


Perinatal mortality increases after 40 weeks, and the rate doubles by 42 weeks; at 44 weeks the perinatal mortality rate is 4-6 times greater than the mortality rate of a term gestation. Maternal, fetal, and neonatal morbidity also increase after 42 weeks.

Maternal Complications. The risk of cesarean birth more than doubles after 42 weeks gestation.

Neonatal complications of post-term pregnancy include macrosomia, shoulder dystocia, brachial plexus injuries, and meconium aspiration.

Confirmation of Gestational Age

Ultrasound Confirmation of Gestational Age. Ultrasound before 24 weeks is the best method of accurately dating a pregnancy. The crown-rump length is the single most accurate predictor of gestational age.

An early pelvic examination (before 12 weeks) with size-dates concordance confirms gestational age.

Clinical Dating

Gestational age is measured from the first day of the last normal menstrual period.

The most reliable clinical predictor of gestational age is an accurately dated first day of last menstrual period. However, the date of last menstrual period is often inaccurate because 20-40% of women cannot recall this date, and even in women who do remember, the date may be unreliable.

Quickening, or the first perception of fetal movement by the mother, occurs at around 19 weeks in first pregnancies and at approximately17 weeks in subsequent pregnancies.

Electronic Doppler can detect