Sickness absence during pregnancy has increased in Sweden as well as in other countries. STUDY OBJECTIVE--The study aimed to describe pregnancy related sickness absence and its increase from 1985-87; to consider if the increase were parallel to an increase in sickness absence for all diagnoses or could be explained by a higher birth rate; and to compare different ways of presenting sickness absence data. DESIGN--The data from a prospective incidence study of all new sick leave spells exceeding seven days in 1985-87 were related to the population at risk through relevant data from different registers. SETTING--The county of Ostergötland, Sweden (about 400,000 inhabitants). PARTICIPANTS--Subjects were approximately 70,000 sick leave-insured women aged 16-44 years, of whom some 15,000 had sickness absences > seven days. Some 4600 women gave birth in 1985, approximately 1300 of whom were listed as having pregnancy related diagnoses. MAIN RESULTS--The number of women with sick leave associated with pregnancy related diagnoses increased by 24% (95% confidence interval (CI) 15, 33%) during the period. This diagnosis group was one of the very few with an increasing number of people listed as sick. The corresponding increase for all diagnoses in women aged 16-44 years was