The use of unfractionated heparin to improve pregnancy outcomes in infertile women has inspired interest in low molecular weight heparin because of several potential advantages. Unfractionated heparin has been shown to inhibit antiphospholipid antibodies (aPL) binding to cardiolipin and phosphatidlyserine in vitro at lower doses than low molecular weight heparin. Low-dose aspirin improves implantation rates by enhancing uterine blood flow. Historically investigators believed that aPL and recurrent pregnancy loss were connected and that treatment with heparin combined with low-dose aspirin was safe and efficacious. Few studies have investigated the role of heparin and low-dose aspirin in aPL positive women undergoing in vitro fertilization (IVF). A systematic review by Nelson and Greer in women with definitive antiphospholipid syndrome or repeated IVF failure suggested that the use of low molecular weight heparin and aspirin started at the time of ovulation induction improved the pregnancy rate in subsequent IVF cycles.