37 weeks is considered full term so I'd at least wait til then, but it doesn't work for everyone. I'm 38 weeks now. Still trying ;)
I started at 37wks and nothing yet. Im 39+5 lol but still doing it hoping it works soon.
Im not sure when people start but I used it and I started labor while I wasz having sex at 39 weeks 4 daysz...of course I kept going lol...