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Table 1 Sample size calculations

From: Maternal health study: a prospective cohort study of nulliparous women recruited in early pregnancy

Exposure comparison (ratio of unexposed: exposed) Outcome of interest Difference Sample size estimate Total number required*
Spontaneous vaginal birth:forceps/vacuumextraction (2:1) Urinary incontinence Faecal incontinence Sexual problems Perineal pain 11% versus 18% 2.6% versus 8% 24% versus39% 20% versus 53% 930 642 360 168 1200 800 450 210
Emergency caesarean:forceps/vacuum extraction (1:2) Urinary incontinence Faecal incontinence Sexual problems 7% versus 18% 3.9% versus 10.5% 29% versus 39% 371 573 841 950 1500 2200
Elective caesarean:forceps/vacuum extraction (1:5) Urinary incontinence Faecal incontinence Sexual problems 2.4% versus 18% 2.8% versus 10.5% 27% versus 39% 276 763 938 900 2500 3100
  1. * Taking into account the relative frequency of different methods of delivery