IntroductionPlacentation disorders are disorders which occur during attachment of the placenta to the uterine wall; their incidence depends on the week of gestation. The incidence of placentation disorders in the 20th week of gestation (GW) is estimated to be around 5 in 100 pregnancies. At delivery, the incidence is only around 0.3 %, as uterine growth in pregnancy often leads to migration of the placenta away from the internal os [1], [2].There are different forms of placentation disorders and they occur with varying degrees of severity, depending on the underlying pathophysiological process. Placentation is a complex process, mediated by prostaglandins, sexual hormones, cytokines and immunological factors [3]. It occurs in the early days after fertilization of the ovum. The fertilized ovum or zygote reaches the uterine cavity around 4 days after fertilization. The zygote divides repeatedly, developing into a blastocyst consisting of two layers. The outer layer of the blastocyst is made up of trophoblast cells which will form the placenta and the fetal membranes. The inner cell layer, the embryoblast, will later develop into the embryo. Physiologically, invasion of the endometrium by the trophoblast is limited to the decidua basalis. Placenta accreta is present when there is excessive invasion during placentation extending beyond the decidua basalis. If invasion by the placenta extends into the uterine myometrium, this is known as placenta increta. If the placental villa penetrate the myometrium and reach the uterine serosa or even invade neighboring organs such as the bladder, this is known as placenta percreta [4]. Rosen has suggested that placentation is regulated by the degree of oxygen saturation of the tissue, with local hypoxia promoting invasion by placental villi. This would explain why scar tissue resulting from previous uterine surgical procedures contributes to placentation disorders [5].In addition to faulty invasion by placental villi, placental disorders include low-lying placenta (placental edge closest to the cervix < 2 cm from the inner os), marginal placenta previa (placenta extends to the edge of the cervix), partial placenta previa (a portion of the cervix is covered by the placenta), and total placenta previa (inner os completely covered by the placenta) [4].
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