The issue of pregnancy post-DVT can be very complicated, and women who have had a previous clotting event need to talk to their doctors about this as soon as possible in their pregnancy. There are a lot of factors to consider when deciding which course to follow, and women should make sure their OB is fully aware of all of the information regarding her case.

Warfarin should be stricly avoided during pregnancy as it is known to cause birth defects. Heparin and low molecular weight heparin (ie Lovenox, fragmin) are considered safe during pregnancy as the molecule does not cross the placenta. However, there are some risks to be considered for the mother if she is on anticoagulation therapy, though most of the time these can be managed. It is important to discuss these risks with your doctor, as well as consider the necessity of receiving anticoagulants as a preventative measure. The studies I've read point to women who are on indefinite anticoagulation therapy as well as those who have thrombophilia as those who should receive preventative anticoags. There is a small study which concluded that women who had a prior clotting episode due to a temporary risk factor (such as injury or surgery) do not need to treated with anticoagulation during pregnancy, as their risk is very low, though they may well want to have testing done to make sure they have no genetic factors affecting their clotting.

Here are two links with information regarding these issues:

http://www.aafp.org/afp/20010515/tips/10.html

http://www.ccjm.org/content/76/2/113.full#T1

Here are some more good links with information/recommendations to consider if you are pregnant and have had DVT in the past. These are just article summaries, but it gives the general recommendations:

http://content.nejm.org/cgi/content/abstract/343/20/1439?ijkey=8c6342270e8e1c24797d5a709cca81d048047e1e&keytype2=tf_ipsecsha

http://www.chestjournal.org/content/133/6_suppl/844S.abstract