Plasma volume expansion is an important component of a successful pregnancy. The failure of maternal plasma volume expansion has been implicated in adverse obstetric outcomes such as pre-eclampsia, fetal growth restriction, and preterm birth. Altered iron homeostasis and elevated maternal hemoglobin concentrations have also been associated with adverse pregnancy outcomes; limited data have suggested that these effects may be mediated by inadequate plasma volume expansion. In addition, it has been noted that pregnant, obese women, compared with lean subjects, have decreased plasma volume expansion along with impaired iron homeostasis and increased inflammation. Current estimates of plasma volume expansion are outdated and do not necessarily reflect contemporary obstetric populations. Moreover, the validation of clinically applicable methods of plasma volume determination as well as enhanced methodologies should be a priority. Further study is needed to characterize diminished plasma volume expansion during pregnancy and to understand the potential role of impaired iron homeostasis and inflammation in adverse obstetric outcomes, especially in obese women.

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Objective The impact of obesity on maternal blood volume in pregnancy has not been reported. We compared the blood volumes of obese and normal weight gravidas using a validated Hydroxyethyl Starch (HES) dilution technique for blood volume estimation. Study Design Blood volumes were estimated in 30 normal weight (pregravid body mass index; BMI < 25 kg/ m2) and 30 obese (pregravid BMI > 35 kg/m2) gravidas after 34 weeks gestation using a modified HES dilution technique. Blood samples obtained before and 10 minutes after HES injection were analyzed for plasma glucose concentrations after acid hydrolysis of HES. Blood volume was calculated from the difference between glucose concentrations measured in hydrolyzed plasma. Results Obese gravidas had higher pregravid and visit BMI (mean [Std Dev]): pregravid (41[4] vs 22[2] kg/m2, p=0.001); visit BMI (42[4] vs 27[2] kg/m2, p=0.001), but lower weight gain (5[7] vs. 12[4] kg, p=0.001) than normal weight women. Obese gravidas had similar estimated total blood volume to normal weight women (8103 ± 2452 vs. 6944 ± 2830 mL, p = 0.1), but lower blood volume per kilogram weight (73 ± 22 vs. 95 ± 30 mL/kg, p = 0.007). Conclusion Obese gravidas have similar circulating blood volume, but lower blood volume per kilogram body weight, than normal weight gravidas near term.