I have had them for both my PG, DS1 being a threatened pre-term labour at 32 weeks, delivered at 34 weeks, baby's lungs were fine.
DS2 I was given them at 30 weeks, due to history, he was delivered at 38 wks.
It is a course of 2x injections 24 hrs apart, make sure it is done 24 hrs apart, if you forget there could be a chance you might need a second course.