Questions about membrane sweeps come up very frequently in our Yoga for Pregnancy groups, as they are routinely offered at term.
The best evidnce on this is the Cochrane Review: Membrane sweeping for induction of labourhttp://onlinelibrary.wiley.com
The authors conclusion is that: routine use of sweeping of membranes from 38 weeks of pregnancy onwards does not seem to produce clinically important benefits. When used as a means for induction of labour, the reduction in the use of more formal methods of induction needs to be balanced against women’s discomfort and other adverse effects.
There are two other Cochrane Reviews on induction which it may be helpful to look at as background to the issue of longer than average pregnancies: Induction of labour for improving birth outcomes for women at or beyond term http://onlinelibrary.wiley.com