From the Guidelines

Yes, enoxaparin (Lovenox) should be switched to unfractionated heparin at 36 weeks of gestation.

The European Society of Cardiology (ESC) guidelines recommend that low molecular weight heparin (LMWH) should be replaced by intravenous unfractionated heparin (UFH) at least 36 hours before planned delivery 1.
This switch is recommended because UFH has a shorter half-life and can be more easily reversed than LMWH, reducing the risk of bleeding complications during delivery.
The guidelines also recommend that UFH should be continued until 4–6 hours before planned delivery and restarted 4–6 hours after delivery if there are no bleeding complications 1.
It is essential to note that the switch from LMWH to UFH should be implemented in a hospital setting, as recommended by the ESC guidelines 1.
The target activated partial thromboplastin time (aPTT) for UFH should be ≥2× control, and the dose should be adjusted accordingly 1.

Switch from enoxaparin to UFH at 36 weeks of gestation
Implement the switch in a hospital setting
Target aPTT for UFH should be ≥2× control
UFH should be continued until 4–6 hours before planned delivery and restarted 4–6 hours after delivery if there are no bleeding complications 1

From the Research

Enoxaparin has several advantages over unfractionated heparin, including a higher ratio of anti-Xa to anti-IIa activity, more consistent release of tissue factor pathway inhibitor, weaker interactions with platelets, and less inhibition of bone formation 2.
Enoxaparin has a higher and more consistent bioavailability after subcutaneous administration than unfractionated heparin, a longer plasma half-life, and is less strongly bound to plasma proteins 2.
Clinical studies have confirmed that these pharmacological advantages translate into improved outcomes, and enoxaparin has been shown to be a safe and effective agent in the treatment of a whole spectrum of acute coronary syndromes, with similar efficacy and safety in the prevention and treatment of venous thromboembolism 3.

There is no direct evidence to suggest that enoxaparin should be switched to unfractionated heparin at 36 weeks of gestation.
However, studies have shown that switching from one anticoagulant to another can be done safely and effectively, with similar outcomes and reduced bleeding complications 4.
The decision to switch from enoxaparin to unfractionated heparin should be based on individual patient factors and clinical judgment, taking into account the patient's medical history, current condition, and potential risks and benefits of each treatment option.

Enoxaparin has a greater bioavailability and longer half-life than unfractionated heparin, permitting less frequent subcutaneous administration 5.
Enoxaparin has been shown to be an effective alternative in the prophylaxis and treatment of thrombosis, with the convenience of less frequent administration than unfractionated heparin and the possible advantage of a lesser propensity for bleeding complications 5.
The pharmacokinetic and pharmacodynamic properties of enoxaparin are distinct from those of unfractionated heparin, and these molecules cannot be regarded as interchangeable 2.