It's almost impossible to find large, randomized studies on factor v leiden and pregnancy. The stakes are high and it is difficult for researchers to recruit. So, even though these studies are small, for those of us who are looking for answers, they are very valuable.
2001 Oxford Journal Human Reproduction showed that the live birth rate for women with FVL AND a history of losses (3 losses or a 2nd or 3rd trimester loss) was even lower than the 49% birth rate for women with similar histories but who had normal Factor V genotype. This study observed pregnancies with no treatment beyond standard prenatal care.
Luckily, treating a clotting disorder can significantly improve the outcomes for women with late or recurrent losses. 2011 Habenox study showed that women with recurrent loss and diagnosed thromobophilia (FVL, Prothrombin, Protein S or C deficiencies, etc.) had approximately 70% live birth rate when treated with 40 mg enoxaparin/lovenox and/or 80mg aspirin daily. Most (90%) of the losses that did occur were 1st trimester miscarriages.
Unfortunately, not all doctors will treat recurrent loss with blood thinners. A study cited by an ACOG practice bulletin of "low risk" women (i.e. no history of recurrent loss) showed that the 134 women with FVL had comparable live birth rates to other low-risk women. For this reason, ACOG does not recommend treating women with FVL unless there is a history of clots. FVL can be very sporadic in it's effects. I had 3 full term births before my diagnosis. But, if it has caused late or multiple losses for a particular mother, the odds are stacked against her for future pregnancies without treating the FVL. (Note: Studies showing the benefits of treatment for recurrent loss are discounted by the ACOG bulletin. Thankfully, many good OBs & Maternal-Fetal medicine specialists DO treat these high-risk populations instead of sitting on their haunches waiting for a large, randomized, double-blind study verifying what more moderately sized studies have already shown- that treating the clotting disorder reduces the risk of further losses.) If you've had multiple or late losses and your doctor cites this practice bulletin as the reason you shouldn't treat your genetic thrombophilia, please find a new doctor.