My ob did a S&S with DS at 41+1 and he was born within 20 hours, but I was 4cm dilated with a favourable cervix - she wouldn't do one until I was over 41 weeks. My midwife ths time is happy to do one from 39+5, someone in my Belly Buddy's group has a midwife who is happy to do it from 38 weeks! So it seems to depend on your caregiver as to when they'll do one.