Serum interleukin-18 (IL-18) levels were measured in women in different trimesters of pregnancy, in labour, puerperium and in various complicated pregnancies. There was a significant elevation of IL-18 levels in pregnant sera from the first trimester until the onset of labour. Once labour began, IL-18 levels increased further and remained until at least the third day of puerperium. The high levels of IL-18 in sera were also observed in various complicated pregnancies, such as premature rupture of membranes (PROM), acute fatty liver of pregnancy and fetal growth restriction. IL-18 is likely to play a role in pregnancy.