My thrombosis specialist is having me switch from warfarin to lmwh as soon as i get a positive pregnancy test. at 4 weeks, is this ok?

No, not okay: You should not be on Coumadin / warfarin at any point in preganncy. It is known to be teratogenic in the first trimester and cause, for example, problems related to limb formation. I would recommend that you schedule a visit with a high-risk OB (called mfm) to discuss transitioning you to lmwh now and to also discuss proper dosing based on your risk factors.

Yes!: Warfarin is not good to take during pregnancy as it can cause birthdefects, lmwh does not have this problem and can be safely used during pregnancy. I would strongly suggest that you seek an opinion from an OB or highrisk pregnancy doctor to be sure you are doing all that they would recommend to have a healthy pregnancy.