ObjectiveTo determine the time course of plasma volume expansion in early pregnancy.MethodsWe 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 withP< .05 accepted for significance.ResultsMean 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.ConclusionPlasma 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.