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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