This chapter deals with the deeper zonal anatomy of the uterus and particularly with the role these areas have in controlling uterine movements. The morphology of the junctional zone (JZ) suggests a distinct compartment of myometrium, tightly packed with muscle cells with an increased vascularity, designed as it were for a specific purpose. The JZ is responsive to changes in sex steroid levels, and undergoes a cycle of change which parallels the changes in endometrial thickness. Rapidly developing technology allows better images and the introduction of ever more complex classification of JZ contractions (JZC). The frequent use of ultrasound during assisted conception cycles provides most of the information we have about JZC, with most data coming from the study of long protocol stimulation in vitro fertilization (IVF) cycles. Excessive JZC have been shown to reduce implantation rates in both spontaneous and stimulated cycles.

At the time of the discovery of the physiological changes of spiral arteries in the pregnant uterus, Brosens and colleagues suggested that these changes result from the destructive action of the invading trophoblasts on the vascular smooth muscle and the elastic membrane. This chapter reiterates the main findings regarding the successive spiral artery remodeling steps. It seems appropriate to relate the time-course of the vascular remodeling process to the new insights in uteroplacental flow changes during this pregnancy period. In preeclampsia, trophoblast-associated remodeling is restricted to decidual spiral arteries throughout the placental bed. Spiral artery conversion is obviously important for safeguarding an adequate maternal blood supply to the placenta. Deep trophoblast invasion and spiral artery remodeling of the inner 'junctional zone' myometrium is a feature of normal human pregnancy, while in preeclampsia and maybe in other pregnancy complications this process may be seriously impaired.