Gestational age and induction of labour for prolonged pregnancy.
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Gardosi J,
Vanner T,
Francis A.
Department of Obstetrics and Gynaecology, Queen's
Medical Centre, Nottingham, UK.
Objectives:
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.
