Blood Thinner During Pregnancy Linked to Better Outcomes in Women Carrying Factor V Leiden Mutation

A new study from Egypt adds fresh fuel to one of the most contested debates in modern obstetrics: whether pregnant women who carry a common inherited blood-clotting mutation should routinely receive anticoagulant drugs, even when they have never suffered a dangerous clot themselves. The research, published in Molecular Biology Reports, followed 200 pregnant women with a confirmed heterozygous Factor V Leiden genotype and found that those treated with antenatal anticoagulation were dramatically more likely to experience a normal pregnancy than those who were not. The treated group achieved normal outcomes in 72 percent of cases, compared with just 24 percent among untreated women, a difference the authors describe as highly statistically significant.

Factor V Leiden is the most prevalent inherited thrombophilia in many populations. It arises from a single nucleotide change, known as G1691A, in the gene encoding coagulation Factor V. That one-letter substitution makes the protein resistant to cleavage by activated protein C, the body’s natural brake on clot formation. As a result, carriers produce clots that persist longer than they should. Women who inherit one mutated copy, the heterozygous state, face a modestly elevated lifetime risk of venous thromboembolism, and that risk climbs further during pregnancy, when the coagulation system shifts naturally toward a pro-thrombotic state to protect against hemorrhage at delivery.

Beyond clotting risk, clinicians have long suspected that the mutation may also sabotage the placenta itself. Microscopic clots in placental vessels can restrict blood flow to the fetus, and this mechanism has been proposed to explain associations between thrombophilia and adverse outcomes such as recurrent pregnancy loss, preeclampsia, placental abruption, fetal growth restriction, and stillbirth. What has remained stubbornly unclear is whether giving blood thinners to women with the mutation, but without a personal history of venous thromboembolism, actually changes those outcomes. International guidelines have hesitated, and the American Society of Hematology’s 2023 guidance on thrombophilia testing reflects a broader skepticism about reflexively testing and treating asymptomatic carriers.

The new case-control study, led by Mai M. Shaker and colleagues at the Human Genetics and Genome Research Institute of the National Research Centre in Giza, set out to test that question directly. The team enrolled 200 pregnant women whose heterozygous Factor V Leiden status had been confirmed by real-time PCR and verified with Sanger sequencing, the gold-standard method for reading out the exact nucleotide sequence at the mutation site. Half of the participants had received antenatal anticoagulation, consisting of low-molecular-weight heparin with or without low-dose aspirin, while the other half had not. The two groups were comparable in baseline characteristics, including age, body mass index, and consanguinity, with no statistically significant differences, which strengthens the argument that the outcome gap reflects the treatment rather than underlying differences between the women.

The headline result is striking. Women who received anticoagulation were 2.71 times more likely to have a normal pregnancy outcome than untreated women, with a 95 percent confidence interval of 1.94 to 3.78. In epidemiological terms, a relative risk of that magnitude, with a confidence interval that excludes one by a comfortable margin, suggests a robust association rather than a statistical fluke. When the researchers dissected the individual adverse outcomes, however, the picture became more nuanced. Only small-for-gestational-age births were significantly reduced in the treated group, with a p-value of 0.02. Other complications, including miscarriage, preeclampsia, and abruption, did not differ significantly between the groups, likely because the number of events in each category was too small to detect a difference even if one existed.

The selective effect on fetal growth restriction fits the biological hypothesis particularly well. If the main harm from Factor V Leiden in pregnancy comes from microvascular clotting within the placenta, then the outcome most directly tied to chronic placental insufficiency, namely poor fetal growth, should be the one most responsive to anticoagulation. Low-molecular-weight heparin does not cross the placenta, making it the anticoagulant of choice in pregnancy, and low-dose aspirin is frequently added when placental dysfunction is suspected. Direct oral anticoagulants, by contrast, are generally avoided in pregnancy, and recent registry data continue to define their bleeding risks in non-pregnant patients with thrombophilia.

Despite the enthusiasm the headline numbers may generate, the authors themselves are careful about what the study can and cannot prove. As a case-control design, the work is observational. Women were not randomly assigned to treatment, and the decision to prescribe anticoagulation may have been influenced by clinical factors, prior pregnancy history, or physician and patient preferences that the baseline comparison of age, body mass index, and consanguinity could not fully capture. Confounding by indication can cut both ways in such studies: treated women might have been selected precisely because they were considered higher risk, which would make the observed benefit even more remarkable, or subtle differences in care and surveillance could have contributed to the outcome gap. The authors explicitly state that confirmation through large prospective randomized trials is required before the findings can translate into routine practice.

That caveat matters because the stakes are real on both sides. Anticoagulation during pregnancy means daily injections of low-molecular-weight heparin for months, along with costs, injection-site reactions, and a small but nonzero bleeding risk, particularly around delivery. Overtreating hundreds of women to benefit a subset would carry its own burden, which is why the field has been reluctant to embrace universal prophylaxis for thrombophilia carriers without prior clotting events. Conversely, for women with a history of adverse pregnancy outcomes and a confirmed heterozygous Factor V Leiden genotype, the possibility that a well-tolerated, placenta-sparing therapy could more than double the chance of a normal pregnancy is exactly the kind of signal that justifies a definitive randomized trial. Prior case-control work, including a 2025 study on low-molecular-weight heparin therapy in thrombophilia carriers, has reported similar directional findings, but the evidence base remains fragmented.

The study also carries implications for genetic screening in prenatal care. Factor V Leiden frequencies vary widely across populations, and prevalence data from countries including Nigeria, Pakistan, and Sudan, cited in the paper’s bibliography, show that the mutation is far from confined to European populations, where it was first described. Consanguinity, which the Egyptian team recorded as a baseline variable, can increase the likelihood of homozygosity for recessive variants and complicate genetic counseling. As genotyping becomes cheaper and more widespread, the question of who should be tested, and what a positive result should trigger clinically, grows more pressing. The authors’ own institution, a prenatal diagnosis and fetal medicine department, reflects the growing integration of molecular genetics into obstetric decision-making.

For now, the study offers a provocative and carefully bounded message: in this cohort of 200 women with heterozygous Factor V Leiden and prior adverse pregnancy outcomes, antenatal anticoagulation was associated with a nearly threefold increase in the likelihood of a normal pregnancy, driven largely by a reduction in small-for-gestational-age births. It is an association, not a prescription. But if larger randomized trials replicate the effect size reported here, one of the longest-running controversies in thrombosis and obstetrics could finally move toward a resolution, and a single-letter spelling change in a clotting gene might no longer cast such a long shadow over pregnancy.

Subject of Research: The effect of routine antenatal anticoagulation on pregnancy outcomes in women carrying the heterozygous Factor V Leiden mutation

Article Title: Association between routine anticoagulation and pregnancy outcomes in women with heterozygous Factor V Leiden genotype

Article References: M. Shaker, M., M. Elaraby, N., & A. shalabi, T. (2026). Association between routine anticoagulation and pregnancy outcomes in women with heterozygous Factor V Leiden genotype. Molecular Biology Reports, 53(1), Article 1599. https://doi.org/10.1007/s11033-026-12686-x

Image Credits: AI Generated

DOI: 10.1007/s11033-026-12686-x

Keywords: Factor V Leiden, thrombophilia, anticoagulation, low-molecular-weight heparin, pregnancy outcomes, placental complications, small for gestational age, venous thromboembolism, genotyping, case-control study, obstetrics, recurrent pregnancy loss