Causes of Moderate Decrease in Prothrombin During Pregnancy
Pregnancy itself is the most common cause of moderate decreases in prothrombin levels due to normal physiological changes in the hemostatic system. These changes occur as part of pregnancy's natural hypercoagulable state, which helps prevent excessive bleeding during delivery.
Physiological Changes in Coagulation During Pregnancy
Normal pregnancy-related changes: Pregnancy induces significant alterations in the coagulation system, with most coagulation factors increasing, but some showing moderate decreases 1.
Hemodilution effect: During pregnancy, plasma volume increases by 40-50% compared to only a 20-30% increase in red cell mass, creating a dilutional effect that can affect coagulation factor concentrations 1.
Prothrombin specifically: While most coagulation factors increase during pregnancy, prothrombin (Factor II) may show relative decreases in some women due to:
- Increased consumption from the hypercoagulable state
- Placental utilization
- Hemodilution from expanded plasma volume
Pathological Causes to Consider
If prothrombin decrease is more significant than expected for normal pregnancy, consider:
Vitamin K deficiency:
- Poor maternal nutrition
- Malabsorption disorders
- Use of certain medications that affect vitamin K metabolism
Liver dysfunction:
- Pregnancy-related liver conditions (intrahepatic cholestasis, HELLP syndrome)
- Pre-existing liver disease exacerbated by pregnancy
Disseminated Intravascular Coagulation (DIC):
- Associated with pregnancy complications like placental abruption, preeclampsia, or amniotic fluid embolism
- Results in consumption of coagulation factors including prothrombin
Medication effects:
- Unintentional exposure to vitamin K antagonists (though these are generally avoided in pregnancy) 1
- Some antibiotics that affect vitamin K-producing intestinal flora
Monitoring and Management Considerations
Serial monitoring: Track prothrombin levels throughout pregnancy, especially in women with known coagulation disorders or liver disease.
Vitamin K supplementation: Consider if deficiency is suspected as the cause.
Risk assessment: Evaluate for additional thrombotic risk factors, as pregnancy itself creates a hypercoagulable state 1.
Delivery planning: For women with significant prothrombin decreases, coordinate with anesthesiology for potential bleeding risk management during delivery.
Important Clinical Pearls
A moderate decrease in prothrombin during pregnancy is often not clinically significant due to the overall hypercoagulable state of pregnancy.
The risk of thrombosis actually increases 5-fold during pregnancy despite these changes in some coagulation factors 2.
Postpartum, there is consumption of platelets and coagulation factors including prothrombin during delivery, with normalization occurring over 4-6 weeks 3.
When evaluating coagulation parameters during pregnancy, always interpret results in the context of gestational age-specific reference ranges rather than non-pregnant values.