The trigger mechanism of parturition is complicated and there are many theories ,so far still no one theory can explain the mechanism perfectly.Currently,lots of researchers tend to think that the parturition is the result of many factors.Throughout the pregnancy process,the expression level of Bcl-2/Bax and Fas/Fasl determines the placental cells growth or apoptosis.Along with the progression of pregnancy,Bax and Fas expression levels are increasing gradually.At the same time,trophoblast cells are decreasing gradually until fetus reaches full-term.This change of placental cells, not only increases the apoptosis of cells, but also reduces the proliferation of cells,which results in the abnormal function and structure of placental and stimu-lates uterine contractions, causing rupture of fetal membranes, promoting the production of oxytocin sub-stance,and leads to the childbirth onset.%分娩动因触发机制复杂，学说众多，至今为止尚没有一个学说能完美阐释其机制。目前众多研究学者倾向于分娩发动是多种因素综合作用的结果。在整个妊娠过程中，Bcl－2／Bax、Fas／Fasl的表达水平高低决定着胎盘细胞的生长或凋亡。随着妊娠的继续，Bax、Fas表达水平逐渐升高，滋养层细胞逐渐减少，一直持续到胎儿足月。胎盘细胞的这种变化，不仅增加了细胞的凋亡，同时也减少了细胞的增殖，从而导致胎盘功能和结构异常，刺激子宫收缩，引起胎膜破裂，促进缩宫物质的产生，导致分娩的发动。
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