It was previously believed that sex differentiation took place when the undifferentiated gonads formed either testes or ovaries. Studies in recent years indicate that sex differentiation begins at conception. The SRY gene on the Y-chromosome is already transcribed at the 2-cell stage and triggers growth acceleration in the XY embryos. This accelerated growth is believed to be important for the male embryo as it allows complete testicular differentiation before the levels of oestrogenic hormones become too high as pregnancy progresses. It is well known that the death rate is higher for male than for female fetuses and that the increase is about 30% in chromosomally normal spontaneous abortions (i.e. significantly higher than at birth). National figures from Sweden show that boys are more likely to be delivered prematurely, accounting for 55-60% of all newborns between 23 and 32 gestational weeks. Neonatal deaths in these gestational weeks are also more common among boys. In 1993, the overall 1-year mortality rate (including all gestational weeks) in Sweden was 5.4% for boys and 4.1% for girls. The difference in infant mortality (within 1 year) is most pronounced at extremely early birth (23-24 gestational weeks) being 60% for boys compared with 38% for girls. The release of catecholamines during labour is an important defence mechanism by a hypoxic fetus. Preterm females have significantly higher catecholamine levels than males, which may explain the better outcome in females after a hypoxic event. Deaths occurring secondary to respiratory distress syndrome are greater for males and their cognitive recovery from perinatal intracranial haemorrhage is worse. Pulmonary hypoplasia after preterm rupture of the membranes is significantly more common among male newborns. Gender differences in mode of delivery, fetal heart rate in labour, acidaemia at birth, and age degenerative changes will also be discussed.