Plasma volume expansion in early pregnancy.

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.