Physiology of the menstrual cycle and natural family planning

Understanding menstrual physiology is the basis of understanding the whole concept of fertility, including the mechanism of action of contraceptives. It is also the basis for understanding natural family planning methods. Hormonal control of ovulation: The menstrual cycle is controlled by the hypothalamo–pituitary axis. The pituitary is a small gland, the size of a cherry, which sits at the base of the brain, anteriorly, behind the bridge of the nose. It is just under the hypothalamus where gonadotrophin-releasing hormone (GnRH), a deca-peptide (a hormone made up of 10 amino acids), is secreted passing into the pituitary via venous channels in a pulsatile manner. It is the frequency and the amplitude of these pulses that determines the response from the pituitary gland, which then secretes follicle stimulating hormone (FSH), the hormone responsible for stimulating the developing Graafian follicles. Follicle stimulating hormone levels are higher in the early phase of the cycle (when it initiates follicular development) than in the later phase. There is a small rise in the level of FSH, which accompanies the very important luteinizing hormone (LH) peak, just prior to ovulation. The anterior pituitary also secretes LH, which remains at basal levels throughout the cycle, with the exception of the LH peak. The rise in the level of LH commences about 36 hours prior to ovulation, and it lasts for 24 hours, with the peak occurring about 24 hours prior to ovulation. This pattern of LH secretion became well understood during the development of the in vitro fertilization programme where women were monitored three hourly in order to detect ovulation, so the oocyte could be collected in a natural cycle and just prior to spontaneous ovulation.