Summary Background Women with certain inflammatory diseases have an increased risk of giving birth to infants who are small for gestational age (SGA) or preterm birth (PTB), with maternal disease activity being the most important risk factor. However, previous studies investigating an association between psoriasis and SGA are scarce and have shown conflicting results. Aim To investigate the association between maternal psoriasis and risk of SGA infants and PTB, respectively, both overall and stratified by psoriasis severity. Methods This was a nationwide register-based matched cohort study of women with psoriasis matched 1 : 10 to women without psoriasis on age at delivery, body mass index and smoking status and with their first singleton infant born during the period 2004–2017. Odds ratio (OR) and 95% CI were calculated in conditional logistic regression models adjusted for known risk factors. Results From 516 063 deliveries, we identified 6282 women with psoriasis and 62 798 matched women without psoriasis. The risk of SGA and PTB was similar in women with psoriasis and matched controls: adjusted OR (aOR) = 1.07 (95% CI 0.98–1.17) and aOR = 1.05 (95% CI 0.93–1.19), respectively. The risk of term SGA was increased in women with psoriasis (aOR 1.11; 95% CI 1.01–1.22) compared with matched controls. Conclusion Maternal psoriasis was not associated with increased risk of SGA or PTB. Risk of term SGA was slightly increased in women with a history of psoriasis compared with matched controls, however; these infants are likely to be constitutionally small with no increased risk of perinatal morbidity and mortality.