Gestational age and induction of labour for prolonged pregnancy.

OBJECTIVE: To examine the length of gestation according to menstrual and ultrasound scan dates, and the rate of induction of labour in a unit with a routine induction policy for prolonged pregnancy.

DESIGN: Retrospective analysis of computer files of 24,675 pregnancies delivered in a teaching hospital between 1988 and 1995, which had a record of the last menstrual period and a dating ultrasound scan. Detailed survey of 168 casenotes of consecutive inductions of labour to establish the indications given.

SETTING: Teaching hospital with policies of routine mid-trimester ultrasound scan and routine induction for prolonged pregnancy at 290 to 294 days.

MAIN OUTCOME MEASURES: Gestational age at delivery by menstrual history and ultrasound biometry in spontaneous and induced labours.

RESULTS: The single largest category of reasons given for induction of labour was prolonged pregnancy. 'Post-term pregnancy', from the date of expected delivery as recorded in the notes, together with 'maternal request' and 'social factors', were the reasons given for induction of labour in 71.3% of cases. Menstrual dates systematically overestimated gestational age at term when compared with scan dates. After 41 weeks, this difference exceeded the confidence limits for second trimester scan dating error, suggesting that most pregnancies which are considered 'prolonged' according to menstrual dates are in fact mis-dated. The median gestational age for induced labours was 286 days by last menstrual period but only 280 days by scan, and most (71.5%) inductions done post-term (> 294 days) according to menstrual dates were not post-term if scan dates alone are used to calculate the gestational age. The average induction rate over the seven year study period was 16.6%. It was higher when there was any gestational age error in either direction (16.8%) compared with when menstrual and scan dates were in complete agreement (13.7%, OR 1.27, CI 1.09-1.47, P < 0.001). The induction rate was highest (up to 21.8%) in the cases where menstrual dates overstated gestational age without exceeding the usual limits for adjusting dates according to scan. Such overestimation within tolerance limits of 7, 10 or 14 days occurred in 37.1%, 45.8%, or 52.6% of all pregnancies, respectively.

CONCLUSIONS: Most pregnancies undergoing post-term induction are not post-term when assessed by ultrasound dates. Regardless of whether prolonged pregnancy is considered to be a risk factor requiring intervention, the proportion of pregnancies considered 'post-term' can be reduced considerably by a dating policy which ignores menstrual dates and establishes the expected delivery date on the basis of ultrasound dates alone.