Yeah, it is kind of ironic that
prometrium has that warning when it is such a common supplement in
early pregnancy. But don't worry, it's really not an issue. Lots and lots of women are using it through the first trimester. As for when to start taking them, it depends on what it is used for. Some women take it every cycle after ovulation if their levels are too low to sustain a possible pregnancy...or to extend a luteal phase defect.