Plasma volume expansion in early pregnancy☆

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Abstract

Objective: To determine the time course of plasma volume expansion in early pregnancy.

Methods: We prospectively measured plasma volume by Evans blue dye dilution during the menstrual (cycle day 2–3), follicular (cycle day 9–10), and luteal phases (cycle day urinary leutinizing hormone [LH] surge plus 9–10) of the menstrual cycle and at three additional time points (LH surge + 16 days, LH surge + 28 days, and LH surge + 70 days) in women achieving pregnancy. Twenty-one subjects were examined during 38 menstrual cycles to establish baseline menstrual cycle data. Ten subjects conceived within 1 year of menstrual cycle studies. All ten pregnancies were viable and reached the third trimester. Analyses used repeated-measures analysis of variance with P < .05 accepted for significance.

Results: Mean plasma volume was found to change significantly across the period of observation (P < .008) in those who conceived. Plasma volume at LH surge + 70 days (12 menstrual weeks, 2320 ± 280 mL) was greater than either menstrual cycle estimates or early pregnancy estimates of plasma volume. There was no difference in plasma volume at LH surge + 16 days (2077 ± 288 mL) or LH surge + 28 days (2010 ± 271 mL) compared with menstrual cycle measurements during the menstrual phase (2156 ± 292 mL), follicular phase (2036 ± 280 mL), and luteal phase (2120 ± 425 mL). There was no significant difference between those who conceived and those who did not in their mean menstrual cycle plasma volume.

Conclusion: Plasma volume expansion in early human pregnancy cannot be identified until after the sixth menstrual week. By 12 menstrual weeks, plasma volume has expanded by approximately 14% ± 12% (mean ± SD) over follicular phase measurements.

Section snippets

Materials and methods

Twenty-two nulligravidas recruited for participation had regular menses (26–35 days), were nonsmokers, were free from major medical illnesses including hypertension, and were interested in becoming pregnant. The women were between the ages of 24 and 35 years and none was taking regular medication. They were enrolled consecutively as a result of open advertisement over the course of 18 months. One subject withdrew from the study after her first cycle because of difficulty in achieving

Results

The 21 subjects had an average age of 29.7 ± 2.3 (mean ± SD) years and a mean follicular phase body weight of 64.5 ± 11.2 kg. The average height was 164.5 ± 5.7 cm and the mean body mass index was 23.8 ± 3.0. Women who conceived were significantly younger than those who did not and had a tendency to be shorter and weigh less (Table 1).

Plasma volume changes beginning with menses and continuing into pregnancy were examined for ten women who conceived. No significant increase in the estimate of

Discussion

The primary objective of this study was to describe the time course of the plasma volume expansion occurring over the first trimester of pregnancy. We found that plasma volume expansion beyond that observed during the menstrual cycle was not observed until after 6 menstrual weeks of gestation. This finding was consistent with the observations made by both Whitaker9 and Lund and Donovan.10 Using Evans blue dilution, Lund and Donovan10 were unable to identify any increase in plasma volume within

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Supported in part by the University of Vermont General Clinical Research Center NIH GCRC MO1-RR109.

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Copyright © 2001 The American College of Obstetricians and Gynecologists. Published by Elsevier Inc. All rights reserved.