From the Guidelines
Blood parameters undergo significant changes throughout pregnancy, with notable alterations in plasma volume, red blood cell count, white blood cell count, and clotting factors, as evidenced by studies such as 1. During pregnancy, various blood parameters change to support fetal development and maternal adaptation.
Changes by Trimester
- In the first trimester, plasma volume begins to expand, while red blood cell production lags, resulting in physiologic anemia with decreased hemoglobin, hematocrit, and red blood cell count, as seen in 1.
- In the second trimester, plasma volume expansion accelerates, further diluting red blood cells and causing hemoglobin to typically drop to 10.5-11 g/dL, with platelet counts possibly decreasing slightly but usually remaining within normal limits.
- By the third trimester, blood volume reaches its maximum expansion (approximately 45-50% above pre-pregnancy levels), and hemodilution is most pronounced, with fibrinogen and other clotting factors increasing significantly to prepare for delivery, as noted in 1.
Key Blood Parameter Changes
- Heart rate increases by 10–20 bpm, particularly in the third trimester, as reported in 1.
- Blood pressure can decrease by 10–15 mmHg by 20 weeks but returns to pre-pregnancy levels by term, according to 1.
- D-dimer levels rise progressively throughout pregnancy, peaking before delivery, although their use is not recommended for diagnostic purposes in pregnancy, as stated in 1.
- Iron requirements increase substantially across all trimesters, often necessitating supplementation to prevent iron deficiency anemia, highlighting the importance of monitoring and managing iron levels throughout pregnancy, as implied by the changes in hemoglobin levels discussed in 1.
These changes represent normal physiologic adaptations that accommodate increased metabolic demands, support placental perfusion, and prepare the maternal system for potential blood loss during delivery, as supported by the most recent and highest quality study, 1.
From the Research
Blood Parameter Changes During Pregnancy
The following blood parameters change during each stage of pregnancy:
- Plasma volume: increases by about 1250 ml, with little increase during the first trimester, followed by a progressive rise to a maximum at about 34-36 weeks 2
- Red cell mass: increases by about 250 ml (18% of non-pregnant volume) in women who take no supplemental iron, and between 400 and 450 ml when iron supplements are taken 2
- Haematocrit: drops from a non-pregnant average of about 40 to about 33 during the last trimester due to the relatively greater increase in plasma volume 2
- Mean corpuscular hemoglobin, mean corpuscular volume, mean platelet volume, platelet count, and red cell distribution width: show significant differences among women in how they change through pregnancy, with trimester-specific associations with postpartum hemorrhage (PPH) 3
- Coagulation factors: increase in thrombin and fibrin generation, necessary for the hemostatic challenge of delivery and normal placental implantation and development 4
- Antithrombin, fibrinogen, prothrombin fragment 1+2, thrombin-antithrombin complex, soluble fibrin, D-dimer, and protein S: show changes throughout pregnancy, with reference ranges established for each stage 5
- Rotational thromboelastometry (ROTEM) parameters: differ across trimesters of pregnancy and compared to the nonpregnant state, with maximum clot firmness and A10 in FIBTEM correlating strongly with fibrinogen level 6
Trimester-Specific Changes
- First trimester:
- Second trimester:
- Third trimester: