Percentage of women who deliver on the expected date of delivery :

Expected date of delivery is calculated using Naegel's rule i.e, 9 months + 7 days calculated from the 1st day of last menstrual period This rule is based on normal 28 days cycle. In case of women with longer cycle additional number of the days that the cycle extends beyond 28 days is added Based on the formula, labour stas approximately : On the expected date - 4% of cases One week on either side - 50% of cases 2 weeks earlier and one week later - 80% of cases Ref: Datta Obs 9e pg 108.