Nattokinase is a serine protease enzyme derived from natto, a traditional Japanese food made from Bacillus subtilis–fermented soybeans. It has attracted attention for its fibrinolytic properties, meaning it can degrade fibrin clots and stimulate the body’s own clot-dissolving pathways. While this activity is the basis of its use in cardiovascular and circulatory support supplements, it also makes nattokinase a substance that warrants serious caution in certain populations.
Pregnancy and breastfeeding represent two of the most medically sensitive periods in a person’s life. During these times, the body undergoes profound changes to its circulatory, hormonal, and immune systems, and the stakes of supplement safety extend beyond the individual to include a developing fetus or nursing infant. No human safety data exist for nattokinase use during pregnancy or lactation, and its known mechanism of action raises several theoretical concerns that are significant enough to make avoidance the clear, evidence-aligned recommendation.
Key Takeaways
- Nattokinase’s fibrinolytic and antiplatelet activity directly conflicts with the protective coagulation changes of pregnancy, creating a theoretical but mechanistically serious bleeding risk.
- No human clinical trials have evaluated nattokinase safety in pregnant or breastfeeding populations; absence of this data is itself a reason for avoidance, not reassurance.
- The supplement is inappropriate during labor, delivery, and any surgical procedure — this risk is compounded for pregnant individuals approaching term.
- Nattokinase may interact with anticoagulants prescribed during pregnancy (such as low-molecular-weight heparin), creating unmanaged additive bleeding risk.
- Anyone using nattokinase who becomes pregnant, or who is planning a pregnancy, should discontinue it and discuss the decision with their healthcare provider.
How Nattokinase Works: A Mechanism Overview
Nattokinase acts directly on fibrin, the protein scaffold of blood clots, cleaving it into soluble fragments. Beyond direct fibrinolysis, it also upregulates endogenous plasminogen activators — proteins that normally help the body dissolve unwanted clots. In addition, some research suggests nattokinase has modest antiplatelet effects, reducing platelet aggregation. These combined actions are why it is studied for conditions involving excessive or inappropriate clotting.
The enzyme is absorbed into the bloodstream following oral ingestion, where it exerts systemic fibrinolytic activity. This systemic reach is precisely why its safety profile matters in pregnancy: any substance that circulates throughout the body and alters hemostasis — the delicate balance between clotting and bleeding — carries implications for the placenta, the developing fetus, and the risk of hemorrhage during labor and delivery.
Because nattokinase is a biologically active enzyme rather than an inert compound, it cannot be treated as nutritionally equivalent to eating natto. Fermented natto itself contains relatively lower levels of the enzyme, while supplements are concentrated preparations standardized in fibrinolytic units (FU). The supplement form therefore presents a meaningfully different exposure than traditional dietary sources.
The Coagulation System Changes Dramatically During Pregnancy
Pregnancy induces a physiologically hypercoagulable state. Concentrations of several clotting factors — including fibrinogen, Factor VII, Factor VIII, Factor X, and von Willebrand factor — rise substantially across all three trimesters. This shift exists for an important biological reason: it prepares the body to rapidly stem blood loss at delivery, when the placenta separates from the uterine wall and large maternal vessels are exposed.
At the same time, the fibrinolytic system is partially suppressed during pregnancy. Levels of plasminogen activator inhibitors increase, particularly PAI-1 and PAI-2 (the latter produced by the placenta). This suppression helps protect the placenta and the implanted embryo from inappropriate fibrinolysis that could disrupt placentation, spiral artery remodeling, or normal placental function.
Introducing a potent exogenous fibrinolytic agent — one that degrades fibrin and stimulates plasminogen activators — directly counteracts these protective adaptations. Disrupting the delicate coagulation balance of pregnancy could theoretically increase the risk of placental abruption, antepartum hemorrhage, miscarriage, or life-threatening postpartum hemorrhage. While direct human data demonstrating these outcomes for nattokinase specifically do not exist, the mechanistic concern is well-founded and sufficient to justify contraindication.
Absence of Safety Data in Pregnant Populations
No published clinical trials have evaluated nattokinase safety in pregnant humans. This is not a gap that should be interpreted charitably: the absence of evidence in a population where research would be ethically complex does not mean the supplement is safe. Standard regulatory and clinical guidance holds that supplements lacking demonstrated safety data in pregnancy should not be used, particularly when the substance has a known mechanism that could plausibly cause harm.
Animal reproductive toxicology studies for nattokinase are also limited in the peer-reviewed literature. Without robust preclinical and clinical data, there is no safety floor — no minimum dose established as harmless, no trimester identified as lower-risk, no route of administration demonstrated to avoid fetal exposure. This complete data vacuum, combined with the active mechanism of the enzyme, is why healthcare providers across disciplines advise pregnant individuals to avoid it entirely.
The FDA has not approved nattokinase for any therapeutic use and has not evaluated it for pregnancy safety. Supplement manufacturers are not required to conduct pregnancy safety studies before marketing their products in the United States. Pregnant consumers and their providers therefore cannot rely on pre-market regulatory review to catch risks — personal vigilance and professional consultation remain the primary safeguards.
Breastfeeding: Additional Unknowns
The safety concerns do not end with delivery. During breastfeeding, substances ingested by the nursing parent can pass into breast milk and be consumed by the infant. Whether nattokinase is excreted into human breast milk has not been studied. Proteins and enzymes vary considerably in their ability to transfer into milk based on molecular weight, protein binding, and other pharmacokinetic factors.
Even if nattokinase were excreted into breast milk, its biological activity in the infant’s body is uncertain. Large protein-based enzymes are often partially or fully degraded in the gastrointestinal tract before absorption, which might limit systemic activity. However, ‘might limit’ is not ‘will not reach the infant,’ and neonates have an immature GI barrier that may allow greater absorption of intact macromolecules than adults. No data exist to resolve this question for nattokinase specifically.
Given the newborn’s developing coagulation system — which is already different from that of healthy adults — any systemic fibrinolytic exposure, even modest, represents an unquantified risk. Until there is evidence demonstrating safety, the conservative and medically appropriate guidance is to discontinue nattokinase supplementation during breastfeeding.
Interaction Risks and Surgical Considerations That Extend to Labor and Delivery
Nattokinase is already contraindicated for use with anticoagulants (warfarin, heparin, low-molecular-weight heparins) and antiplatelet agents (aspirin, clopidogrel) due to additive bleeding risk. In pregnancy, low-molecular-weight heparin is frequently prescribed for conditions such as antiphospholipid syndrome, thrombophilia, or mechanical heart valves. A pregnant person taking a prescribed anticoagulant who also takes nattokinase faces a compounded, unmanaged bleeding risk.
Healthcare providers typically advise that nattokinase be discontinued at least one week before any surgical procedure due to its fibrinolytic and antiplatelet activity. Labor and delivery — whether vaginal or by cesarean section — involves significant vascular events. Cesarean delivery is major abdominal surgery with real hemorrhage risk. Even vaginal delivery involves tearing of small vessels and the rapid separation of the placenta. Carrying active fibrinolytic enzyme activity into delivery is not acceptable from a surgical safety standpoint.
Epidural and spinal anesthesia, commonly used in labor, also carry their own bleeding risk at the site of needle placement. Anesthesiologists conducting neuraxial procedures follow strict guidelines about discontinuing anticoagulants and blood-thinning supplements in advance. Nattokinase would fall within the class of substances requiring discontinuation before neuraxial anesthesia, adding yet another operational reason why the supplement is inappropriate in late pregnancy.
Evidence-Based Alternatives and Practical Guidance
If a pregnant or breastfeeding individual is interested in nattokinase because of concerns about clotting, circulation, or cardiovascular health, the correct path is to discuss those underlying concerns with an obstetrician or maternal-fetal medicine specialist — not to self-manage with supplements. Conditions such as deep vein thrombosis and pulmonary embolism in pregnancy are managed with medications that have established safety profiles, appropriate dosing guidelines, and monitoring protocols.
General cardiovascular health during pregnancy is supported by evidence-based lifestyle approaches: regular moderate physical activity as tolerated and approved by a provider, a diet rich in whole foods, appropriate weight management, and adequate hydration. None of these carry the mechanistic risks of a potent fibrinolytic enzyme.
For those who used nattokinase before becoming pregnant, the appropriate action upon discovering pregnancy is to stop the supplement and inform their obstetric provider at the earliest opportunity. There is no established washout time specific to nattokinase in pregnancy; the provider can advise on monitoring if there are concerns about recent use.
🛒 Where to Buy Nattokinase
- Doctor’s Best Nattokinase 2,000 FULab-tested / studied
capsules, 100 mg NSK-SD per vcap (2,000 FU) — Most widely referenced brand in clinical and integrative medicine contexts; uses Japan Bio Science Laboratory NSK-SD ingredient; vegetarian capsules; 90 count - NOW Foods Nattokinase 100 mg
capsules, 100 mg per vcap (2,000 FU) — Mainstream GMP-certified brand; affordable entry-level option; 90 vcaps; widely available - Source Naturals Nattokinase 100 mg
capsules, 100 mg per tablet (2,000 FU) — Long-established supplement brand; competitive pricing at 60 tablets; good for budget-conscious buyers familiar with the brand - Healthy Origins Nattokinase 2,000 FU
capsules, 100 mg per vcap (2,000 FU) — Best cost-per-serving option on Amazon; 180 vcap bottle; uses NSK-SD ingredient; popular bulk buy for long-term users
As an Amazon Associate we earn from qualifying purchases. Shilajit quality varies widely — always choose a product with a published third-party heavy-metal test (COA) before buying.
A Note on the Evidence
This article is informational only and does not constitute medical advice. Nattokinase has not been evaluated by the FDA for safety in pregnancy or breastfeeding, and no clinical trials have studied these populations; all mechanistic concerns described here are based on the enzyme’s known pharmacology, not documented human outcomes in pregnant individuals. Anyone who is pregnant, breastfeeding, or planning a pregnancy should consult their obstetrician or qualified healthcare provider before taking any supplement, including nattokinase.
Frequently Asked Questions
Is nattokinase safe to take in early pregnancy, such as the first trimester?
No trimester has been established as safe for nattokinase use. No clinical trials exist in any stage of pregnancy, and the mechanism — systemic fibrinolytic activity — is relevant throughout gestation. Early pregnancy includes the critical period of implantation and placentation, during which disruption of normal hemostasis could be particularly consequential.
Can I take nattokinase to prevent blood clots during pregnancy?
Pregnancy-related clotting disorders are managed with medications that have established safety data and clinical monitoring protocols, not with over-the-counter supplements. If you have a diagnosed or suspected clotting condition, speak with your obstetrician or a hematologist who specializes in pregnancy. Self-treating with nattokinase in this context would be unsafe.
If I accidentally took nattokinase before knowing I was pregnant, what should I do?
Inform your obstetric provider as soon as possible. Single or short-term unintentional exposure is unlikely to cause a documented harm, but your provider should be aware and can decide whether any monitoring is warranted based on the dose, duration, and your individual pregnancy context.
Does eating natto — the traditional fermented food — carry the same pregnancy risks as taking nattokinase supplements?
Traditional natto contains far lower levels of nattokinase enzyme than concentrated supplement preparations. However, the research comparing fermented food intake to supplement-level fibrinolytic exposure in pregnancy has not been conducted. Natto is a well-established part of Japanese dietary culture, but pregnant individuals should discuss any dietary concerns with their provider, particularly if consuming natto frequently or in large quantities.
Could nattokinase in breast milk harm my nursing baby?
It is not known whether nattokinase is excreted into human breast milk. If it were consumed by an infant, the biological activity would depend on intestinal absorption, which may differ in neonates compared to adults. Because newborns have a developing coagulation system and no safety data exist for this exposure scenario, the precautionary recommendation is to avoid nattokinase during breastfeeding.
Are there any supplements proven safe for cardiovascular support during pregnancy?
Some nutrients, such as omega-3 fatty acids (particularly DHA) and magnesium, have been studied in pregnancy and are used under provider supervision. However, no supplement should be started during pregnancy without explicit provider approval, because safety depends on the specific compound, dose, trimester, and individual health history. Your obstetrician is the appropriate person to advise on this.
These statements have not been evaluated by the Food and Drug Administration. This information is not intended to diagnose, treat, cure, or prevent any disease. Content is for informational purposes only and is not medical advice; consult a qualified healthcare provider before starting any supplement. As an Amazon Associate we earn from qualifying purchases.
