A new formula for the prenatal estimation of the weight of extremely preterm fetuses was derived using ultrasound measurements and birth weights of 73 premature infants delivered before 30 completed weeks of pregnancy and weighing between 400 and 1,680 g at birth. The actual birth weight lay within +/- 15% of the estimated weight calculated with this formula in 92% of the cases. Preliminary testing of the reliability of the formula was performed on a further test group of 19 nonselected cases. The formula developed in this study offers a reliable and simple method of prenatal estimation of fetal weight between 23 and 30 weeks of gestation.