What does factor V Leiden mean for pregnancy?

What does the evidence say about factor V Leiden during pregnancy?

1. Factor V Leiden changes a protein involved in clotting. Some carriers never develop a clot. A result describes inherited susceptibility, not a current deep-vein clot or an inevitable pregnancy complication, and it needs to be interpreted alongside the personal history. 1
2. One-copy and two-copy results have different implications. Decisions about anticoagulation also account for previous thrombosis, combined clotting variants and other clinical risks. A positive genetic report alone is not enough to copy another patient’s injection schedule. 2
3. Pregnancy and the period after birth add clot risk through changes in clotting and blood flow. Discuss prolonged immobility, cesarean birth, family history and any previous clot with the maternity team. Ask what symptoms and delivery circumstances should trigger reassessment. 3
4. ASRM’s recurrent-pregnancy-loss guidance does not recommend routine inherited-thrombophilia testing as a general explanation for repeated losses. Factor V Leiden is distinct from antiphospholipid syndrome. Testing and medicines should answer a defined clinical question rather than promise an improved outcome from a genetic result alone. 4
5. If anticoagulation is indicated, the team selects an appropriate pregnancy regimen and plans around birth and postpartum care. Do not start, stop or replace a prescribed blood thinner independently. Reassess the plan when circumstances change, including after delivery. 2

Questions for your clot-risk consultation

• First trimester. Bring your exact genetic result and details of any previous blood clot.
• Second trimester. Ask how delivery, procedures or new immobility would change the plan.
• Third trimester. Confirm postpartum follow-up and the symptoms that require emergency help.