This prospective randomized trial will compare outcomes in patients receiving aspirin in combination with a low-molecular weight heparin and those under this center's standard protocol receiving neither medication, in order to determine if these medications may improve success rates.

This prospective randomized trial will compare outcomes in patients receiving aspirin (81mg daily) in combination with a low-molecular weight heparin (LMWH) (enoxaparin (Lovenox®), 40mg daily subcutaneous injection) and those under this center's standard protocol receiving neither medication, in order to determine if these medications may improve success rates.

Inclusion Criteria:

1. Female patient 18-42 years of age with frozen embryos or oocytes seeking embryo transfer.
2. Use of pre-implantation genetically tested (PGT) blastocysts is allowed but not required.
3. Use of donated embryos or embryos derived from donated eggs is allowed.
4. Prior history of successful, failed, and/or canceled IVF cycles are allowed.

Exclusion Criteria:

1. Minor (age<18 years).
2. Currently pregnant.
3. Unable to provide informed consent in English.
4. Gestational carrier or "surrogate".
5. Blastocysts frozen at another center (Oocytes frozen elsewhere is acceptable).
6. Subject intending or having "natural cycle" thaw transfer with an active ovarian follicle present.
7. Currently participating in any other research study.
8. Subject already had an embryo transfer under this study.
9. History of thrombocytopenia (reduced platelets), bleeding disorders, or routinely using anti-coagulant medication.
10. Hypersensitivity to aspirin, heparin, or benzyl alcohol.
11. Anyone for whom the physician assesses this protocol is inappropriate or unsafe.

Female seeking pregnancy.