Birth weight curves according to gestational length are presented based on data from more than 480,000 singleton births, registered in the Medical Birth Registry and with gestational age based on ultrasound examinations in the majority of cases. Curves were constructed from the most common weights in each week (modes) for each sex and standard deviations were estimated under the assumption of a fixed coefficient of variation, the size of which was estimated from 40 weeks infants. This methodology makes it possible to construct graphs for specific subgroups of infants: such graphs for boys and girls for primiparous and multiparous women are given in the paper.