Fourteen days before the next period is due, the dominant follicle releases its egg at ovulation. The follicle changes its structure and becomes a corpus luteum (Figure 2.3) in the luteal phase of the menstrual cycle. It continues to function but now it not only releases oestrogens but also progesterone. The progesterone causes a change in the lining of the womb from a proliferative to a secretory pattern. In pregnancy, the lining of the uterus changes its structure to become the decidua which in turn develops into the placenta or afterbirth. Progesterone (pro -in favour and gestation pregnancy) is essential for the development of the decidua. The decidua produces a hormone called HCG (human chorionic gonadotrophin). This HCG is essential to keep the corpus luteum functioning. A cycle is set up with the decidua producing HCG which results in the corpus luteum producing progesterone and this in turn maintains the decidua. From about the twelfth week of pregnancy, the placenta has formed and is self-sufficient in the production of progesterone.
