the prophylactic dose or preventative dose for clexene in pregnancy is 40mg and unless you have multiple clotting problems most people are on this. 2 or more clotting issues will get you 60 or 80mg which is a therapeutic dose. Have read studies comparing the outcomes of being on 80 v 40 and there is no increased live birth rate with the higher dose unless u have the multiple clotting problems. Basically 40mg is the dose you should be taking in pregnancy to prevent problems. 20mg is very little and even my previous FS had me on the 40mg. I would definitely be following Dr M and taking the 40mg. I have started at D1, ovulation and on BFP so I wouldn't worry too much about what day once your on it when you get a BFP.