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Does a first trimester dating scan using crown rump length measurement reduce the rate of induction of labour for prolonged pregnancy? An uncompleted randomised controlled trial of 463 women.

Harrington DJ, MacKenzie IZ, Thompson K, Fleminger M, Greenwood C

Department of Obstetrics and Gynaecology, The Women's Centre, John Radcliffe Hospital, Oxford, UK.

OBJECTIVE: To evaluate the effect of a first trimester ultrasound dating scan on the rate of induction of labour for prolonged pregnancy. DESIGN: Randomised controlled trial to include 400 women in each arm of the trial. SETTING: Participating general practices and a district general teaching hospital. POPULATION: Women attending their general practitioner in the first trimester to confirm pregnancy, in whom a first trimester ultrasound scan was not indicated. METHODS: Women randomised to the study group (scan group) underwent an ultrasound dating scan between 8 and 12 weeks, measuring crown-rump length. The estimated date of delivery (EDD) was changed if there was a discrepancy of more than 5 days from the gestation, calculated from the last menstrual period (LMP). For the remaining women (no-scan group), gestation was determined using the LMP. MAIN OUTCOME MEASURES: The rate of induction of labour for prolonged pregnancy. RESULTS: Due to circumstances beyond the researchers' control, recruitment was abandoned when 463 women had been enrolled. The EDD was adjusted in 13 (5.7%) women in the scan group and in 2 (0.9%) in the no-scan group. There was no difference in the rate of induction for prolonged pregnancy between the scan (19 [8.2%]) and the no-scan (17 [7.4%]) groups (relative risk 1.10; 95% CI 0.59-2.07). CONCLUSIONS: Acknowledging the reduced numbers recruited for study, it is concluded that there is no evidence that a first trimester ultrasound dating scan reduces the rate of induction of labour for prolonged pregnancy and may result in a more expensive healthcare strategy.

Published 17 January 2006 in BJOG, 113(2): 171-6.
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