Studies on pregnancy and birth outcomes in women with psoriasis are scarce and the findings inconsistent. The effect of maternal psoriasis and its severity on the risk of adverse pregnancy and birth outcomes was examined. A cross-national population-based cohort study was performed using prospectively collected data from Denmark and Sweden. Singleton births in women with psoriasis were identified in the national health registers between April 2007 and December 2012 and classified according to disease severity. Lifestyle factors, co-morbidity, pregnancy and birth outcomes were evaluated. A total of 8,097 births were identified in 6,103 women with psoriasis and 964 births in 753 women with psoriatic arthritis. Increased risk of gestational diabetes, gestational hypertension, pre-eclampsia, elective and emergency caesarean delivery was found in women with psoriasis. The risks were higher for women with severe psoriasis, who also had an increased risk of pre-term birth and low birth weight. Pre-conception counselling to improve maternal, pregnancy and birth outcome is encouraged.