Maternal Hb levels during the third trimester were studied in relation to certain maternal and fetal parameters in 877 apparently normal pregnancies. Low Hb levels at term were closely associated with increased frequency of newborns in the heavy weight-for-date group. Conversely, high maternal Hb levels were closely associated with an increased frequency of newborns in the light weight-for-date group. The maternal Hb levels both in the early third trimester and at term were significantly higher in mothers of small-for-date newborns than in those with newborns of normal weight. In both groups the maternal Hb levels increased significantly during the third trimester of pregnancy. High maternal Hb levels both early and late in the third trimester of pregnancy should be a matter of concern rather than of reassurance.