HMOX1 and preeclampsia: The limitations of our study include not collecting sequential blood mainly after the development of preeclampsia; the small number of severe and mild preeclampsia cases, increasing the chance of alpha type error; measurement of plasma HO-1 level, and this represents a sum of all HO-1 tissue production; not measuring HO-1 level before 20 weeks of gestation, which may show us other results; and when the study was conducted.