Hemoglobin levels during twin vs. singleton pregnancies. Parity makes the difference.

A retrospective, case-control study of 200 consecutive twin and singleton gestations matched for parity was conducted to challenge the hypothesis that lower hemoglobin values appear more often during twin gestations. The database consisted of hemoglobin levels recorded during each trimester. Comparisons were made between the mean hemoglobin levels and incidence of values > 11, 9-11 and < 9 g/dL. Significantly lower mean (+/- SD) hemoglobin values were observed during the first (11.94 +/- 1.1 vs. 12.17 +/- 1.0, P = .026) and second (11.1 +/- 1.0 vs. 11.48 +/- 0.9, P = .00001) trimesters in twin vs. singleton pregnancies, respectively. The differences resulted from lower values in multiparas with twins as compared with their singleton-pregnancy controls (11.8 +/- 1.2 vs. 12.2 +/- 0.9, P = .015, for the first trimester and 11.0 +/- 1.0 vs. 11.5 +/- 0.9, P = .0001, for the second trimester). During the second trimester, the lower incidence of values > 11 g/dL (P = .005, odds ratio = .475, 95% confidence interval = .31-.73) and higher incidence of values between 9 and 11 g/dL (P = .0008, odds ratio = 2.06, 95% confidence interval = 1.34-3.19) may account for the significant differences in mean values. There were no differences between third-trimester values, between nulliparas in both groups or between nulliparas and multiparas in each group. We conclude that the lower hemoglobin levels in twin gestations were associated with multiparity and were statistically significant during the first and second trimesters. This subgroup of twin pregnancies may benefit from further research and possibly from closer hematologic care and monitored iron supplementation.