The data of the 2,040 single births, born during 1987 at the "Saint Antoine" Hospital in Paris, were analysed in order to identify the impact of various biological, occupational, and socioeconomic factors on gestational age and birth weight. Birth weight is associated with the height of the mother and the weight gained during pregnancy. It is lower for mothers with preeclampsia during the current or previous pregnancies or with urogenital infections during the current pregnancy and for mothers with one or more induced abortions. Girls weigh less than boys. Parity has a positive relation to the baby's weight, while manual work seems to have a negative one. APGAR score and duration of the pregnancy are associated with the birth weight. Placenta previa, preeclampsia and urinary infections affect the gestational age. A short pause period in work is related to a shorter gestational age. Weight gain is associated with a prolonged duration of the pregnancy. Gestational age and birth weight are associated with the nationality of the mother, especially in some ethnic groups, and with marital status.