A linear perinatal growth curve, covering the period from 28 weeks gestation to 18 weeks after term birth is presented. The curve was first presented 15 years ago, and is now supported by Danish data, both cross-sectional and longitudinal. It may be used for classification of birth weight in relation to gestational age and at the same time for monitoring of postnatal growth, regardless of gestational age at birth. Limitations of empirical curves are pointed out. The postnatal growth potential of preterm infants is stressed.