Normal Fibrinogen Concentrations in Second and Third Trimester of Pregnancy
Normal fibrinogen levels in the second trimester range from 340–850 mg/dL (3.4–8.5 g/L), and in the third trimester from 370–620 mg/dL (3.7–6.2 g/L), with a median of approximately 386 mg/dL at 36 weeks gestation. 1
Physiologic Elevation Throughout Pregnancy
Fibrinogen rises progressively from the first trimester and continues to increase throughout gestation, reflecting heightened coagulation factor production as term approaches 1
This elevation represents a normal hypercoagulable adaptation that protects against postpartum hemorrhage rather than indicating pathology 1
Mean fibrinogen levels exceed the conventional non-pregnant reference interval as early as the 16th week of pregnancy 2
Trimester-Specific Reference Ranges
Second Trimester (13–24 weeks)
- Reference range: 340–850 mg/dL (3.4–8.5 g/L) 1
- Mean levels in uncomplicated pregnancies: approximately 404 mg/dL (4.04 g/L) 3
- Alternative reported range: 260–656 mg/dL (2.60–6.56 g/L) in early-to-mid pregnancy 4
Third Trimester (25–42 weeks)
- Reference range: 370–620 mg/dL (3.7–6.2 g/L) 1
- Median at 36 weeks: 386 mg/dL (3.86 g/L) 5, 1
- Mean levels in uncomplicated pregnancies: approximately 440 mg/dL (4.40 g/L) 3
- Alternative reported range: 363–914 mg/dL (3.63–9.14 g/L) 4
Clinical Interpretation Thresholds
Pathologically Low Levels
Fibrinogen < 300 mg/dL (< 3 g/L) is a key criterion in pregnancy-specific disseminated intravascular coagulation (DIC) scoring systems 5, 1
Fibrinogen ≤ 200 mg/dL (≤ 2 g/L) has a 100% positive predictive value for severe postpartum hemorrhage 5, 1
Fibrinogen < 150 mg/dL (< 1.5 g/L) in the second trimester warrants fibrinogen replacement therapy in women with congenital hypofibrinogenemia to prevent placental abruption 5, 1
Target Levels for High-Risk Conditions
Women with congenital fibrinogen disorders should maintain trough levels ≥ 100 mg/dL (≥ 1 g/L) throughout pregnancy, or 50–100 mg/dL (0.5–1 g/L) during the first two trimesters if higher targets are difficult to achieve 5
In cases of vaginal bleeding or placental abruption, target fibrinogen levels should be increased to ≥ 150 mg/dL (≥ 1.5 g/L) 5
Important Clinical Caveats
Elevated fibrinogen in pregnancy denotes normal physiology and increased thrombotic risk—it does not signify bleeding risk 1
Only critically low fibrinogen levels (approximately < 150–200 mg/dL) are predictive of hemorrhagic complications 1
The percentage of high molecular weight fibrinogen remains unaltered during normal pregnancy despite the increase in total fibrinogen concentration 6
Women with pregnancy-related complications show significantly higher mean fibrinogen levels throughout gestation, except in placental abruption where levels decrease from the third trimester and may fall below those of uncomplicated pregnancies 3
Gestational diabetes mellitus (GDM) is associated with significantly higher fibrinogen concentrations compared to uncomplicated pregnancies 4
budget:token_budget Tokens used this turn: 4857 Total tokens used: 4857 Tokens remaining: 195143