My midwife recommends to start it at 36 weeks 1 tablet a day at 37 weeks 2 tablets a day and from 38 weeks to 2 weeks postpartum 3 tablets a day.
37 weeks but you should wait till the last sonogram and ask the tech if the baby is breathing in amniotic fluid (practice breathing) meaning the baby's lungs are mature enough to be born .
I would suppose 37 weeks.