Objectives: Mask-wearing can induce relative hypoxia, affecting hematologic parameters. We aimed to evaluate the effect of the COVID-19 mask-wearing on hematological laboratory components and obstetrical outcome among women delivering during this period. Design: Retrospective cohort study Setting: Tertiary medical center. Population: Pregnant women delivering a singleton gestation. Methods: Obstetrical outcome and laboratory results of women admitted for delivery throughout the mask-wearing period were compared to those delivering during the parallel period in 2019, and to a larger cohort derived from nine pre-pandemic years Main outcome measure: Hematological, delivery and neonatal outcomes characteristics. Results: Overall, 1,939 women delivered during the COVID-19 pandemic. Compared with the pre-pandemic period, the mean hemoglobin and fibrinogen levels were significantly higher during the mask-wearing period [12.15±1.1 vs. 11.96±1.2, p<0.001 and 472±103.6 vs. 448±85.1 (mg/dL), p<0.001, respectively]. Platelets levels were lower [200±56.0 vs. 206±57.5 (K/microL), p<0.001]. The rate of delivery<34 weeks of gestation was lower during the mask-wearing period [1.1% vs. 2%, OR-0.57 (95% CI, 0.37-0.88), p=0.01], while cesarean delivery and postpartum hemorrhage rates were higher [[26.7% vs. 24.4%, OR- 1.13 (95% CI, 1.02-1.25), p=0.022 and 4.1% vs. 2.8%, OR- 1.5 (95% CI 1.2-1.8), p=0.001, respectively], Conclusion: A hard-to-ventilate space created by wearing a mask may be the underlying cause of the observed higher hemoglobin level among pregnant women. This change might have a biological relation to changes in the rate of obstetrical outcome. Funding: no external funding. Keywords: COVID-19, preterm birth, cesarean delivery, composite neonatal outcomes, hemoglobin, fibrinogen, platelets, mask-wearing.