The length of human pregnancy is variable, reflecting the advantages to the fetus, which would benefit by staying in the uterus to grow more, and to the mother, for whom earlier delivery might reduce pelvic damage (see the second article in the series). The result of this interaction is a relatively high incidence of premature deliveries. In the United Kingdom the incidence of preterm delivery (before 37 weeks) is about 7%, and in many developing countries, it is much higher. The baby is delivered before its homoeostatic mechanisms are properly developed and so is prone principally to the respiratory distress syndrome, hypothermia, hypoglycaemia, and jaundice. Distribution of 24 675 deliveries by gestational age (determined by early ultrasound scanning dates) for a British maternity hospital (Queen's Medical Centre, Nottingham), 1988-95 Socioeconomic factors influence the incidence of preterm labour. Preterm birth is significantly more common in young women, those with low body weight (body mass index <19), those of lower social class, unmarried or unsupported mothers, and smokers. Some medical factors may increase the risk of preterm birth—for example, previous preterm delivery, persistent vaginal bleeding in early pregnancy, and heart disease. Cervical incompetence is a rare cause of preterm labour, sometimes preventable by cervical cerclage (a purse string suture around the cervix as close to the internal os as possible). The Medical Research Council's trial showed a small but significant benefit from this procedure, even in women with equivocal risk factors. ### Major risks of early preterm delivery ### Risk factors for preterm labour ### Major causes of preterm labour Infection, usually …