Risks of delivery during the 20th to the 36th week of gestation.

Using routinely collected maternity discharge data from 250,000 women in Scotland, we examined the risks of late spontaneous abortion and preterm delivery during the period 20 up to 36 weeks of gestation. Gestational age is treated as a fetal survival time and the risks of delivery associated with a history of spontaneous abortion, induced abortion and perinatal death are examined in a survival model controlling for several demographic and socioeconomic variables. The main objective of the study is to identify factors which are associated with high relative hazard of delivery early in the period, but with decreasing relative hazard of delivery as pregnancy progresses. The factor most clearly associated with converging hazards is a history of two or more spontaneous abortions, and this may reflect the tendency to repeat pregnancy outcome.