Tried to do one at 40+4 with ds2 but cervix was high hard and closed. He was born 5 days later within 3hrs.
I had one at 38 and 38+5 with Dd. The first one gave me cramps but did nothing real. The second one didn't do a thing. I was induced the next day (my induction internal I was 2-3cm dilated, cervix was still pretty posterior and thick. Same as the day before . Do the s&s did nothing). Labour was less than 2hrs.
Eta: I had IDGD and they wanted to induce at 38wks, but I refused and asked for s&s instead. That's the only reason they were done before 40wks. They would not do it before 38wks.