Background: Red blood cells are primarily formed in the bone marrow and their production is highly affected by pregnancy. Obstetric hemorrhage is the leading cause of maternal death and morbidity. The effect of pregnancy on the hemoglobin concentration at the different gestational age was not properly identified especially in resource limited settings. The objectives of this study were to estimate and identify the determinants of hemoglobin concentration before pregnancy, during pregnancy and after labor and delivery.

Methods: Prospective cohort study design was implemented. Data were collected from May 2015 to September 2018.epi-info software was used to calculate the sample size. Simple random sampling technique was used to select the study participants. Interview technique was used to collect the data. Blood samples were collected before pregnancy, during each trimester, during labor and delivery, after third stage of labor and at the 6 weeks post partum period. Descriptive statistics were used to describe the profile of study participants. Generalized estimating equation (GEE) was used to identify the determinants of hemoglobin concentration during each phase of pregnancy.

Results: A total of 1709 study participants were completely followed giving for the response rate of 88 %. The mean hemoglobin concentration of primigravid and multigravid before pregnancy were 12.41 g/dl and 10.79 g/dl respectively. The hemoglobin concentration decreases with consecutive trimester reaching the lowest level during labor and delivery, iron deficiency anemia was the leading type of anemia ranging from 63.9 % to 94.6 % of all form of anemia. The hemoglobin concentration of pregnant women was decreased by hookworm 0.24g/dl [95% CI:0.18-0.29], Multiple pregnancy 0.16 g/dl [95% CI: 0.07- 0.24],  Episiotomy 0.05g/dl [95% CI: 0.01-0.09], gravidity 0.15g/dl[95%CI: 0.09-0.21], age 0.03g/dl [95%CI: 0.03-0.04], gestational age0.1g/dl[95% CI: 0.09-0.11]. The hemoglobin concentration increased by iron supplementation 1.02g/dl [95% CI: 0.97 -1.07], birth weight 0.14 g/dl [95% CI: 0.02-0.11], induction 0.09g/dl [95% CI: 0.02-0.14].

Conclusion: pregnancy significantly decreases the hemoglobin concentration of pregnant women reaching the lowest point during labor and delivery.

Keywords: hemoglobin concentration; variation; pregnancy; Ethiopia