Erythropoietin levels rise from early in pregnancy and by term are approximately double the level seen at initial booking. This results in an 18–25% increase in the red cell mass which meets the increased maternal oxygen requirements during pregnancy. The rise in red cell mass is even greater in multiple pregnancies. The plasma volume expands by 50% in pregnancy, however, and therefore, despite this increase in red cell mass, a physiologic fall in hemoglobin concentration is seen during pregnancy. The hemoglobin concentration reaches a nadir at about 20 weeks gestation and increases slowly from about 30 weeks until term. It is thought that the lower hematocrit may result in lower blood viscosity which aids placental perfusion and oxygenation of the fetus. The degree of plasma volume expansion is strongly associated with fetal size [1]. Maternal hemoconcentration (determined by an elevated hematocrit reflecting inadequate plasma volume expansion) is associated with premature labor, intrauterine growth restriction and pre-eclampsia.

The World Health Organization has recommended that hemoglobin levels during pregnancy should ideally be maintained at or above 11 g/dL (6.83 mmol/L) compared to 12 g/dL (7.44 mmol/L) in nonpregnant women. The US Centers for Disease Control and Prevention (CDC) considers anemia to be defined by a hemoglobin of <11g/dL in the first and third trimesters and <10.5 g/dL in the second trimester. Other investigators report that the lower limit of normal variation of hemoglobin levels is about 10 g/dL (6.2 mmol/L), with the nadir at around 25 or 26 weeks gestation [2,3]. While anemia is common in pregnancy, it is important to distinguish between physiologic anemia, which is harmless, and pathologic anemia, which may cause harm and/or represent a manifestation of an underlying disease.

Several investigators have found an association between iron deficiency anemia, detected early in pregnancy, and adverse pregnancy outcome. The data are most compelling for cases of severe maternal anemia (i.e. hemoglobin levels less than 6 g/dL) and include increased risks of preterm delivery, low birthweight and both perinatal and maternal mortality. While similar associations have been reported in some studies of women with less severe anemia, the data do not support a clear causal link. It is important to consider that it may be the factors causing the maternal anemia (i.e. infection, chronic illness, hereditary conditions and/or nutritional deficiencies) rather than the anemia per se that are responsible for pregnancy complications. Studies on the efficacy of iron supplementation to reduce the incidence of low birthweight and preterm delivery have been limited and inconclusive thus far, although there are no clear contraindications to routine supplementation in most pregnant women.

With respect to neonatal effects, there is increasing evidence that maternal iron deficiency may result in lower infant iron stores, an outcome which can be prevented by adequate maternal iron supplementation during pregnancy [4,5].