Warfarin is teratogenic to a fetus. so why would my thrombosis specialist tell me to switch to lmwh only when i get pregnant and not before? confused!

Teratogenic to fetus: If you get pregnant, you should switch to lmwh -the moment you know you are prgnant, as you don't want the Coumadin (warfarin) to harm the baby. However if you are not pregnant- you should be fine with coumadin (warfarin). Many times, eventhough you try to get pregnant, nobody knows when or whether you are going to be pregnant, could be months, a year etc. Lmwh is injection, expensive, other side effect~long use of lmwh.

Why treated?: We need to know the reason for warfarin. If you have a metal heart valve replacement, there is no good alternative to warfarin.

It is teratogenic: Your physician is right in switching you from warfarin to lmwh when he /you know that there is pregnancy . This is because as you rightly said warfarin is teratogenic . Since you have thromboembolic condition that require anticoagulation , you need to be on a safe medication . It is the standard of care to put pregnant women on lmwh until birth . You were not on lmhw earlier because it is the right.

Days of exposure: I'm assuming that your doctor knows you will take a pregnancy test as soon as you miss a period and that your periods are regular. There's no harm in switching to lmwh before conception, if you feel strongly, you can suggest that. The 2012 accp guidelines only 'weakly' suggest switch from warfarin --> lmwh after conception. There's absolutely no problem on lmwh while trying to conceive.