Objective To investigate the incidence and risk factors for neonatal seizures in a nationwide cohort of infants born between 1990 and 2005.

Method Data were retrieved from the medical birth register (MBR) and the Swedish hospital discharge register, 1 613 328 births were evaluated.

Results The overall incidence of neonatal seizures was 2.1 per 1000 live-born infants with no change over time. The incidence of neonatal seizures was lowest (1.7/1000) at 37–41 gestational weeks (GW) and increased in both preterm (21.9/1000 at 25–28 GW) and post term deliveries (3.0/1000 at ⩾42 GW). Infant risk factors included: male gender (odds ratio (OR) 1.33; 95% CI 1.25 to 1.43) and (U-shaped associations) pre and postmaturity, birth weight (BW) <2500 g and ⩾4000 g and BW deviation >1 SD from the expected. Maternal risk factors included: primiparity (OR 2.04; 95% CI 1.88 to 2.21), grand multiparity (OR 1.17; 95% CI 1.01 to 1.37), obesity (pre-pregnancy body mass index >30) (OR 1.72; 95% CI 1.54 to 1.92) and smoking (>10 cigarettes per day) (OR 1.29; 95% CI 1.11 to 1.49). Maternal diabetes and preeclampsia remained as significant risk factors also when data were adjusted for maternal age, parity, smoking, gestational age and BW: diabetes (OR 3.27; 95% CI 2.53 to 4.23) and preeclampsia (OR 1.37; 95% CI 1.16 to 1.61). The risk of neonatal seizures increased with maternal age (p<10–6).

Conclusion There has been no change in the incidence of neonatal seizures in Sweden from 1990 to 2005. Prematurity, postmaturity, low BW, high BW, male infant gender, maternal diabetes, preeclampsia, obesity and heavy smoking are all significant risk factors for the development of neonatal seizures.