From the Guidelines
Maternal blood volume increases by approximately 30-50% during pregnancy. This significant increase begins early in the first trimester and continues progressively until it peaks at 20 to 24 weeks of gestation 1. The expansion consists of both plasma volume and red blood cell mass. According to the most recent and highest quality study, plasma volume reaches a maximum of 40% above baseline at 24 weeks gestation 1.
Key Points
- The increase in blood volume is essential to meet the increased metabolic demands of pregnancy, including supplying adequate oxygen and nutrients to the growing fetus, placenta, and maternal tissues.
- The disproportionate increase in plasma compared to red blood cells leads to the physiologic anemia of pregnancy, characterized by a decrease in hemoglobin concentration despite an absolute increase in red blood cell mass.
- This blood volume expansion also helps protect the mother against the normal blood loss that occurs during delivery.
- The hemodynamic changes associated with pregnancy impose a marked hemodynamic stress in women with certain medical conditions, such as pulmonary arterial hypertension, leading to a substantial mortality risk 1.
Hemodynamic Changes
The hemodynamic changes during pregnancy include:
- Increase in cardiac output (CO) by 30-50% 1
- Reductions in systemic vascular resistance and blood pressure (BP) 1
- Increase in stroke volume and heart rate 1
- Decrease in diastolic BP by 10 mmHg below baseline in the second trimester 1
From the Research
Maternal Blood Volume Increase During Pregnancy
- The increase in maternal blood volume during pregnancy is a significant physiological change, with studies indicating a range of increases:
- A study published in 2017 2 found that plasma volume increased by 45.6% (95% CI, 43.0-48.1%) in physiological pregnancies compared to the non-pregnant state.
- Another study from 2019 3 reported that plasma volume increased by 48% (95% CI, 44-51%) in weeks 35-38 of gestation.
- A 1985 study 4 estimated that plasma volume increases by approximately 50% during a normal pregnancy, with an average increase of about 1250 ml.
Patterns of Plasma Volume Expansion
- The pattern of plasma volume expansion across gestation varies, with:
- The steepest increase occurring during the second trimester, according to the 2017 study 2.
- The highest rate of increase occurring in the first half of the second trimester, as reported in the 2019 study 3.
- A progressive rise in plasma volume until about 34-36 weeks, followed by little or no further increase, as described in the 1985 study 4.
Comparison of Studies
- While the studies differ in their estimates of plasma volume increase, they all indicate a significant expansion of maternal blood volume during pregnancy.
- The 2017 study 2 and the 1985 study 4 provide similar estimates of plasma volume increase, while the 2019 study 3 reports a slightly higher increase.
- The studies from 2019 3, 2022 5, and 2019 6 do not directly address the question of maternal blood volume increase, but rather focus on the relationship between anemia and adverse pregnancy outcomes.