Nulliparity is a significant risk factor for pre-eclampsia because the maternal immune system is exposed to paternal antigens for the first time during the initial pregnancy. This lack of prior immune tolerance can lead to abnormal placental development and endothelial dysfunction, which are central to the pathophysiology of pre-eclampsia. Multiparous women generally have developed tolerance due to previous pregnancies, reducing their risk. Therefore, nulliparity increases the likelihood of abnormal immune adaptation and pre-eclampsia.

Reference: Williams Obstetrics, Cunningham et al., 26th Edition.