• Eligibility Criteria:
• Inclusion (must meet all three criteria):
• 1. Subjects with a prior adverse outcome in a prior pregnancy. Adverse outcome is defined as prior singleton preterm birth ( \< 37 weeks), SGA infant (defined as birthweight \< 10th percentile), preeclampsia with severe features, or stillbirth (fetal demise after 20 weeks gestation), as certified by an obstetrician
• 2. Patients with maternal vascular malperfusion on pathology from pregnancy with prior adverse pregnancy outcome, as certified by a perinatal placental pathologist
• 3. Current singleton pregnancy at \<16 6/7 weeks gestational age.

Exclusion Criteria:

• 1. Anticoagulation planned for current pregnancy (including warfarin, enoxaparin, heparin)
• 2. Known major fetal anomaly
• 3. Contraindication to enoxaparin: Specifically active major bleeding, known thrombocytopenia (platelets \<100), hypersensitivity to enoxaparin sodium, hypersensitivity to heparin or pork products, hypersensitivity to benzyl alcohol
• 4. Chronic kidney disease with eGFR\< 60
• 5. Known chronic liver disease with baseline AST/ALT \> 3 x upper limit of normal
• 6. Subjects with mechanical prosthetic heart valves