*2.2.1 Failure of step 1: decidua-associated remodeling is defective*

Late luteal phase secretory endometrium decidualization is associated with infiltration of natural killer cells, which are now considered to be major effector cells at trophoblast-uterine interphase interactions. It has also been postulated that repeated cycles of menstrual shedding of decidualizing endometrium may act as preconditioning for successful implantation and deep placentation [13]. This might explain the increased risk of preeclampsia in teenage pregnancy, short interval of pregnancy since menarche, and primipaternity. This may also explain the lowered risk of preeclampsia in women who have intercourse earlier with partner who fathers the current pregnancy. Recent research also suggests that natural killer cells' associated defects of implantation are due to disturbed ligand receptor interphase. Uterine natural killer cells are absent in the JZ myometrium, but their angiogenic action is mediated by interstitial trophoblast.
