Hematologic profiles in twin vs. singleton pregnancies across gestation

While changes in hematologic parameters are well characterized in singleton pregnancies, less is known about twin gestations. Most prior studies have focused on hemoglobin and hematocrit, with limited data on other complete blood count (CBC) values. As CBCs are routinely used in prenatal care, understanding how they differ in twin pregnancies is important for accurate interpretation and clinical decision-making.

To compare CBC parameters between twin and singleton pregnancies at three gestational time points and evaluate how these values change across gestation by pregnancy type.

This retrospective cohort study included 419 pregnancies (185 twins, 234 singletons) delivered between 2019 and 2024 at a large maternal-fetal medicine practice. All eligible twin pregnancies were included. Singleton controls were randomly sampled to achieve approximately similar representation to twin pregnancies at early (≤14 weeks), mid (>14 to <30 weeks), and late (≥30 weeks) gestational time points. Pregnancies with conditions or medications affecting CBC values were excluded. Mean CBC values were compared using independent two-sample t-tests and linear regression models adjusted for gestational age at testing. Changes from early to late pregnancy were also compared between groups.

In early pregnancy, white blood cell (WBC) count was higher in twin pregnancies (9.4 ± 2.0 vs. 8.6 ± 2.3 × 10³/µL; adjusted p < 0.001), with no other significant differences. This persisted through mid pregnancy, when twins also had lower hemoglobin, hematocrit, and red blood cell counts. By late pregnancy, WBC levels no longer differed, while platelet count was lower (193.9 ± 49.9 vs. 206.4 ± 51.3 × 10³/µL; p = 0.019) and red cell distribution width higher in twins (13.8 ± 2.0% vs. 13.4 ± 1.2%; p = 0.015). WBC increased over time in singletons but remained stable in twins (p = 0.001).

Twin pregnancies exhibit distinct hematologic patterns compared to singletons, including elevated WBC counts beginning early in gestation and a more pronounced platelet decline. These findings suggest an inherent hematologic difference in multifetal pregnancies and support the need for twin-specific reference ranges to guide interpretation of CBC values in prenatal care.