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To record the frequency of sonographic reassignment of the EDD and to assess the effect of this on obstetric outcome in women delivering beyond 41 weeks′ gestation.
A consecutive observational cohort study over 12 months to June 2008, of singleton cephalic nulliparas after 41 weeks′ gestation (term plus 7 days). Dates were assigned following a scan performed around 20 weeks′ gestation. Induction of labor was offered at 42 weeks′ gestation. .
Among 4233 primiparas (> 37 weeks), 1000, 23.6% delivered after 41 weeks′ gestation. Overall the EDD was reassigned in 406 cases (40.6%); put back in 297 cases (29.7%) and brought forward in 109 cases (10.9%). A potential indication for alteration of the EDD pertained in 497 cases (49.7%) resulting in a reassignment in 352 cases (72%) compared with 8.5% (43 of 505) among those with certain dates. Labor was spontaneous in 594 cases (59.4%), at a rate of 10 % per day after 41 weeks′ gestation and the caesarean rate (13.6%; 81 of 594) was constant. Among those whose labor was induced for post-term pregnancy, the overall caesarean delivery rate was 33.7% (126/373) and was greater among those with altered dates (36% 52/144 vs. 32.3%: 74/229) P=0.5 and highest 51 %(21/41) when the EDD was brought forward, compared with those put back (29% 30/103; P<0.01). The onset of labor was not influenced by maternal age, BMI or birth weight.
The EDD was reassigned sonographically in a relatively high proportion of all cases (40.6%) but did not affect the caesarean rate among women who labored spontaneously (13.6%). The caesarean delivery rate for induced labor (33.7%) was higher and particularly when the EDD had been altered. Caution should be exercised when planning induction of labor for uncomplicated post term pregnancy when dates have been reassigned by mid-trimester ultrasound.