Objective: To reevaluate the concept that poor maternal hematologic changes relate to increased placental protein hormones, increased birth weight, and placenta to birth weight ratio.
Methods: Sixty-nine normal women were studied prospectively. On several occasions, pre-pregnancy, during pregnancy, and post-delivery, plasma volume was measured together with maternal hematologic indices and placental protein hormone levels. Birth weight and placental weight were measured at delivery.
Results: The decrements in hemoglobin concentration and hematocrit were apparent by 7 weeks' gestation. By 12 weeks, these progressive changes resulted mostly from the increase in plasma volume, and both hemoglobin concentration and hematocrit continued to decrease until near term. Increments in red cell and hemoglobin mass were maximal at 12-28 weeks of pregnancy. Concentrations of the placental hormones hCG and human placental lactogen at 12 weeks showed a lack of correlation with hemoglobin concentration or any other hematologic index. The correlations of birth weight with hemoglobin concentration and hematocrit at 36 weeks were not significant when we controlled for the effect of plasma volume. Neither hemoglobin mass at 36 weeks nor the change in hemoglobin concentration, hematocrit, and mean cell volume from the pre-pregnancy value to that at 36 weeks were significantly related to birth weight, placental weight, or the placenta to birth weight ratio.
Conclusion: Low hemoglobin in late pregnancy reflects plasma volume changes, rather than poor maternal nutrition or adaptation, and is not linked to discordant placenta to birth weight ratio.