I get a fair few questions about whether or not to go for a sweep in my Doula work. It isn't straight forward with studies varying in quality, but I'll share what I know.
Generally, having a sweep MAY decrease the length of pregnancy by 1-4 days. It doesn't mean that a woman will go in to labour 1-4 days after a sweep, but she may give birth a few days before she would have without having it.
As a method of induction it isn't very reliable and only works in roughly 10% of women. That means that 9 women out of 10 will still need to have further intervention to begin labour (assuming it's medically necessary for her to be induced).
70% of women find the procedure very painful and she needs to be at least 1cm dilated for it to be carried out. Just the exam to find out if she is indeed 1cm can be very painful and it may be that the sweep can't be carried out because her cervix is firmly closed. For some women this results in a disappointment as she may have believed that it can always be done, but also that it would mean that her baby will be here sooner.
When the sweeps are carried out at 40 weeks, the results are varying. One study on 195 women concluded that only 8% of the sweep group needed a formal induction because of post dates, compared to 18% in the control group and that it therefore was a useful procedure. I personally don't think that 1 extra woman not needing an induction makes up for the discomfort (and maybe disappointment) for the other 8. But that's for every woman to decide on her own. Two studies (one smaller study of 120 women and one larger of 200) concluded that sweeps at 40 weeks does not reduce the need for formal inductions at 42 weeks and that the discomfort needs to be taken into account when offered.
Often sweeps are routinely offered at 39 weeks so I looked into what sweeps do if offered then. One study concluded that that women who were routinely swept from 38 weeks did not have any clinical benefits, didn't change the due date and didn't reduce the needs for formal inductions.
The Cochrane review in which all studies on sweeps were looked at concluded: "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."
A study in which 741 women who were 41 weeks pregnant had sweeps every 48 hours showed that it can be a useful intervention to prevent going past 42 weeks, which is when women are recommended to have a formal induction. 1/5 women still needed a formal induction in the sweep group compared to 2/5 in to non sweep group.
Personally I take this information to mean that the more ready a baby is to be born, the more significant effect does the sweeps have. I'm not sure as to where the recommendation comes from to have a sweep at 39 weeks, maybe it's just to coincide with a normal midwife appointment as resources are scarce? I struggle to find any convincing evidence that it will actually work the way it supposed to which is to reduce formal inductions at 42 weeks. However I find the results for sweeping every 48 hours from 41 weeks convincing more convincing. It also makes more sense that the body would respond better if baby is getting to be born anyway.
All studies have concluded that sweeping is a safe option though, so if a woman is well informed and understands the it can be possibly very painful (more so for first time mothers) and incur bleeding she can choose this options without being worried for serious side effects. Also, she doesn't have to start worrying about going into labour soon after, so she can carry on as she was. Equally a woman can decline to have it done and simply wait a couple of weeks.