Complex sentence: The prevalence of postpartum psychosis in the general population is 1--2 per 1,000 childbirths, but the rate is 100 times higher in women with bipolar disorder or a previous history of postpartum psychosis.
Split into simpler sentences: The prevalence of postpartum psychosis in the general population is 1--2 per 1,000 childbirths. However, the rate is 100 times higher in women with bipolar disorder or a previous history of postpartum psychosis.