Activated partial thromboplastin time (aPTT)
Pregnancy is a hypercoagulable state. Increased levels of factors VIII, IX, X, and fibrinogen cause a mild shortening of the aPTT as gestation advances, although values generally remain within the normal adult range.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| seconds | 26.3–39.4 | 24.3–38.9 | 24.2–38.1 | 24.7–35.0 |
Pregnancy physiology
- Pregnancy increases procoagulant factors (VIII, IX, X, fibrinogen).
- Mild physiological shortening of aPTT occurs with advancing gestational age.
- aPTT typically remains within the normal reference range.
- Significant prolongation should not be attributed to pregnancy and warrants evaluation.
Causes of prolonged aPTT
- Heparin therapy (including LMWH contamination)
- Direct thrombin inhibitors (dabigatran, argatroban)
- Factor VIII, IX, or XI deficiency
- Von Willebrand disease (severe forms)
- Lupus anticoagulant / antiphospholipid syndrome
- Acquired factor inhibitors
- Advanced liver disease
- Disseminated intravascular coagulation (DIC)
- Massive transfusion
Causes of shortened aPTT
- Acute phase reaction (↑ factor VIII)
- Pregnancy-related hypercoagulability
- Malignancy
- Early DIC (hypercoagulable phase)
- Technical artifact (partial clotting before analysis)
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–31.
- Wallach J. Interpretation of Diagnostic Tests, 8th ed.
- Fischbach FT, Dunning MB. A Manual of Laboratory and Diagnostic Tests, 7th ed.