 As per routine care, the patient will be instructed by the clinic nurse on how to fill her prescription at the pharmacy, how to give herself the injection subcutaneously once daily in the morning, and how to troubleshoot the injection process. As per routine care, the patient will have the opportunity to ask all questions necessary about this process, and her understanding and ability to comply with the injection procedures will be assessed by the clinic RN. The patient will be given a new prescription as needed by her physician at her regularly scheduled MD appointment. At any time, as per routine care, the patient can have the opportunity to review or troubleshoot the injection process with the RN or MD in the clinic. At each clinic visit, as per routine care, approximately 1-2 times per month as per routine care, the patient will be weighed and her weight recorded in her prenatal chart. At each of these visits the patient will also be asked a series of questions as per the study flowsheet in the chart, about potential side effects or adverse events she may have experienced since her last visit. In addition, she will receive a phone call from one of the study investigators once to twice per month to enquire about side effects, medication tolerability, and medication compliance. At 3 predetermined intervals throughout the pregnancy, the patient will be instructed to give a blood sample at the outpatient laboratory. When at all possible, these blood draws will be coordinated to coincide with the patients regularly scheduled prenatal care blood draws, to minimize venipuncture episodes. The study blood sample results will test for: anti-Xa level , which is a marker of the effective prophylactic range of Enoxaparin, serum creatinine, which is a marker of kidney function and renal clearance of medication, and also for coagulation analysis using TEG to assess the coagulation effects of enoxaparin. TEG is a point-of-care device which measures the viscoelastic properties of clot formation. This device can provide rapid and detailed information about coagulation changes. The anti-XA level sample results will be blinded to the medical care providers and to the patient, and used for research purposes only. At or shortly after the patient reaches 36 weeks gestation, she will be switched from enoxaparin to unfractionated heparin UFH, as per standard of care to avoid any potentially unexpected bleeding events associated with the onset of labor at term. When the patient comes to the hospital for Labor and Delivery, routine labor and delivery care will ensue at the discretion of the medical care team. 24 hours after delivery, or at the discretion of the medical care team, the patient will be restarted on her Enoxaparin dosing arm, as per standard of care. She will be instructed to continue the enoxaparin for 6 weeks, and will receive a prescription from the in-hospital providers for this. The patient will return for a 6 week follow-up visit in the clinic as per standard of care, and a final study blood draw will be performed at that time. 