Prothrombin Time (PT)
Prothrombin Time — Trimester-Specific Reference Ranges
PT evaluates the extrinsic and common clotting pathways (factors I, II, V, VII, X). During pregnancy, increased hepatic production of clotting factors—especially factors VII and X— leads to a **slight physiologic shortening** of PT.
| Units | Nonpregnant | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| seconds | 12.7–15.4 | 9.7–13.5 | 9.5–13.4 | 9.6–12.9 |
Physiologic changes in pregnancy
- ↑ factor VII and X production → mild PT shortening
- ↑ fibrinogen and overall hypercoagulable state
- Pregnancy itself does not cause PT prolongation
- Any elevated PT warrants evaluation for coagulopathy
Causes of shortened PT (low PT)
- Physiologic pregnancy changes (↑ factor VII & X)
- Hypercoagulable states
- Early disseminated intravascular coagulation (DIC)
- Excess tissue factor release
- High factor VII activity
Causes of prolonged PT (high PT)
- Vitamin K deficiency
- Liver dysfunction or hepatic failure
- DIC (consumptive phase)
- Warfarin exposure
- Massive hemorrhage / dilutional coagulopathy
- Factor deficiencies: I, II, V, VII, X
- Malabsorption of fat-soluble vitamins
- Severe sepsis
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table. Obstet Gynecol. 2009;114:1326–31. PMID: 19935037
- James AH. Pregnancy-associated thrombosis. Hematology Am Soc Hematol Educ Program. 2009.
- Leung LLK. Coagulation changes in pregnancy. UpToDate.