When is labor supposed to start after last 17p injection?

[brenda3]

I had my last 17p injection when I was 35 weeks and 1 day. I am now 36 weeks and 4 days and starting have contractions yesterday (not regular times 20 min apart, 30 min apart ect)
Does anyone know when you normally go into labor after your last 17p injection? I also have a cerclage in and do not want it to tear my cervix with the contractions, but high risk ob said call if contractions are regular every 15 min or severe bleeding.
I am sch. for my c-section at 39 weeks and 1 day.

Any help !!!!!!