| Summary: | Abstract Background Plasma volume expansion is an important physiologic change across gestation. High or low expansion has been related to adverse pregnancy outcomes, yet there is a limited understanding of normal/healthy plasma volume expansion. We aimed to evaluate the pattern of plasma volume expansion across healthy pregnancies from longitudinal studies. Methods We conducted a systematic review and meta-analysis to identify original studies that measured plasma volume in singleton pregnancies of healthy women. Specifically, we included studies that measured plasma volume at least two times across gestation and one time before or after pregnancy in the same women. PubMed, Web of Science, Cochrane, CINAHL, and clinicaltrials.gov databases were searched from the beginning of each database to February 2019. We combined data across studies using a random effects model. Results Ten observational studies with a total of 347 pregnancies were eligible. Plasma volume increased by 6% (95% CI 3–9) in the first trimester compared to the nonpregnant state. In the second trimester, plasma volume was increased by 18% (95% CI 12–24) in gestational weeks 14–20 and 29% (95% CI 21–36) in weeks 21–27 above the nonpregnant state. In the third trimester, plasma volume was increased by 42% (95% CI 38–46) in weeks 28–34 and 48% (95% CI 44–51) in weeks 35–38. The highest rate of increase occurred in the first half of the second trimester. Included studies were rated from moderate to high quality; 7 out of 10 studies were conducted over 30 years ago. Conclusions In healthy pregnancies, plasma volume begins to expand in the first trimester, has the steepest rate of increase in the second trimester, and peaks late in the third trimester. The patterns observed from these studies may not reflect the current population, partly due to the changes in BMI over the last several decades. Additional longitudinal studies are needed to better characterize the range of normal plasma volume expansion across maternal characteristics.
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