Estimation of fetal size by ultrasound has supported two recent concepts of fetal growth. Firstly, the normal weight gain is constant from 28 weeks of gestation until several weeks after term; about 27 g/day. Secondly, the average weight of fetuses born preterm is lower than the average weight of fetuses of the same gestational age who remain in utero. This means that up to 40% of infants born at 28-30 weeks of gestation are small-for-gestational age, when related to the biological optimum. Unfortunately, the test-retest variation of fetal weight estimation is as much as 50% of the standard deviation of the gestational-age-specific fetal weight distribution. This means that fetal growth has to be followed for many weeks to document statistically significant deviations. No study has, as yet, demonstrated the value of this approach to distinguish between intrauterine growth retardation and growth along low centiles by fetuses of reduced growth potential.