The relationship between haemoglobin values (14th to 30th week of gestation), pregnancy outcome and perinatal morbidity was investigated in a prospective study. Subsequently, haemoglobin values, blood pressure, proteinuria and perinatal risk factors, together with the foetal cardiotocogram were abstracted from the obstetrician's records. Preterm birth (25%), intrauterine growth retardation (7.6%) gestational hypertension (31.5%) and low birth weight babies (10.3%) were seen significantly more often in women with haemoglobin > or = 13 g/dl in the 2nd trimester (14-30 wk). We observed a high perinatal morbidity from RDS (9.3%) and newborn hyperviscosity (23.9%) in women with a high haemoglobin level. These results were in agreement with the hypothesis, that a higher blood viscosity or a lack of haemodilution are risk factors for poor placental perfusion.