Anticoagulants are medications used to reduce the blood’s tendency to form clots. Treatment is often necessary during pregnancy because the body naturally enters a hypercoagulable state. This physiological change, intended to prevent excessive bleeding during delivery, significantly increases the risk of dangerous blood clots, such as deep vein thrombosis (DVT) or pulmonary embolism (PE), by four to five times compared to non-pregnant women. Because management is complex, treatment must be managed by a multidisciplinary team, including an obstetrician, a hematologist, and potentially a maternal-fetal medicine specialist. The timing of when to start these medications is highly individualized and depends entirely on a person’s specific risk factors and medical history.
Identifying Conditions Requiring Therapy
Anticoagulant therapy during pregnancy is primarily initiated to prevent Venous Thromboembolism (VTE) or complications in high-risk patients. A prior history of VTE is the most significant risk factor for recurrence, especially if the event was unprovoked, meaning it occurred without a clear, temporary trigger like surgery or trauma. The substantial risk of recurrence necessitates preventative treatment throughout gestation and into the postpartum period.
Certain inherited or acquired blood disorders, known as thrombophilias, also mandate anticoagulant use. Inherited conditions like Factor V Leiden mutation, Prothrombin G20210A mutation, and Antithrombin deficiency increase the tendency toward clotting. The risk level dictates the specific protocol, with Antithrombin deficiency carrying one of the highest risks for a thrombotic event.
An acquired autoimmune disorder called Antiphospholipid Syndrome (APS) requires prophylactic treatment, often involving anticoagulants and low-dose aspirin. APS is linked to adverse pregnancy outcomes, including recurrent miscarriage and pre-eclampsia, due to placental vascular issues. Additionally, women with mechanical heart valve prostheses are at high risk for valve thrombosis and systemic emboli, requiring therapeutic-level anticoagulation to protect the mother and the pregnancy.
Determining the Optimal Initiation Time
High-Risk Initiation
For women considered high-risk, such as those with a mechanical heart valve or severe thrombophilia like Antithrombin deficiency, treatment often begins before or immediately upon a positive pregnancy test. Patients already taking warfarin must discontinue it early due to fetal risks. The goal in these cases is to switch to a fetus-safe anticoagulant within the first six weeks of gestation to avoid teratogenic exposure.
Prophylactic Timing
For women with a history of VTE that was unprovoked or occurred during a previous pregnancy, prophylactic treatment is started as early as practical in the first trimester. Patients with moderate risk, such as those with a single prior VTE event related to a transient risk factor, may also begin prophylactic dosing early in the antenatal period.
Acute VTE Treatment
If a new VTE event occurs during pregnancy, the timing and dosage change significantly, requiring a shift from prophylactic (preventative) dosing to therapeutic (treatment) dosing. Therapeutic doses are substantially higher and are weight-adjusted to actively dissolve or halt the growth of an existing clot. Treatment for acute VTE is initiated immediately upon diagnosis and continues throughout the remainder of the pregnancy.
Types of Anticoagulants Used in Pregnancy
Low Molecular Weight Heparin (LMWH)
LMWH is the preferred anticoagulant for most pregnant individuals and is considered the standard of care. Its large molecular structure prevents it from crossing the placenta, making it safe for the developing fetus. It is administered via subcutaneous injection, usually once or twice daily, and offers predictable drug levels with less need for frequent blood monitoring.
Unfractionated Heparin (UFH)
UFH is another heparin product that does not cross the placenta. It is typically reserved for specific high-risk situations, such as when rapid reversal of anticoagulation is necessary or close to the time of delivery. UFH has a shorter half-life than LMWH, allowing for quicker discontinuation if an urgent procedure or delivery is required.
Warfarin
Warfarin, an oral anticoagulant, is generally avoided during the first trimester because it can cross the placenta and is associated with birth defects (warfarin embryopathy). However, in specific, high-risk scenarios, such as for women with certain mechanical heart valves, warfarin may be used in the second and third trimesters. This is because it offers the lowest maternal risk of valve thrombosis. The risk to the fetus must be carefully weighed against the higher risk of a life-threatening clot for the mother, requiring specialized, shared decision-making.
Managing Treatment Through Delivery and Postpartum
Delivery Planning
As the delivery date approaches, anticoagulation must be managed to minimize maternal bleeding and allow for regional anesthesia. Women on prophylactic LMWH are typically advised to stop their medication approximately 12 hours before a planned induction or Cesarean section. For those on a higher, therapeutic dose, LMWH must be held for at least 24 hours prior to a scheduled delivery or anticipated time of neuraxial anesthesia.
Bridging and Anesthesia
If labor begins spontaneously while the patient is on therapeutic LMWH, or if immediate reversal is necessary, the patient may be transitioned to intravenous UFH. This strategy, known as bridging therapy, uses UFH because it has a faster and more predictable reversal agent. An epidural or spinal anesthetic cannot be safely administered until a specified time interval has passed since the last dose of LMWH or UFH to prevent a spinal hematoma.
Postpartum Continuation
The risk of VTE remains highest in the postpartum period, often extending for the first six to twelve weeks after delivery. Therefore, anticoagulant therapy must be continued for at least six weeks postpartum, and often for a minimum of three months, depending on the initial indication. LMWH is often continued throughout this period, but switching back to an oral anticoagulant like warfarin is also common, as both are generally considered safe while breastfeeding.