The human female is monotocous. The complex and highly regulated sequence of events that manifests as regular monthly menstruation exists to ensure that only one oocyte is ovulated in any one cycle and that implantation of an early embryo can arrest the process of endometrial shedding and ensure its survival. Monthly menstruation is an obvious marker that the various levels of interaction between hypothalamus, pituitary, ovary and uterus are functional. Interruption of this axis at any point leads to disordered menses. Gynaecologists will frequently have to investigate and treat such disorders. A clear understanding of the regulation of the normal cycle is therefore necessary to guide rational management when things go wrong.
Although termed the 'menstrual cycle', since menstruation is the obvious monthly event during reproductive life, the normal menstrual cycle is mostly a reflection of ovarian events. The selection and growth of the dominant follicle leads to increasing concentrations of oestrogens in the blood, stimulating endometrial growth. Later, following the luteinizing hormone (LH) surge, ovarian oestrogens and progesterone from the corpus luteum induce endome-trial secretory changes and the decline in luteal steroid production in the absence of pregnancy leads to the onset of menstruation. Hence a description of clinical relevance of the menstrual cycle should focus on ovarian physiology, while not overlooking events in the hypothalamus and pituitary and at the level of the uterus.
The menstrual cycle is regulated at both endocrine and paracrine levels. Endocrinologically, there are classical feedback loops that modulate release of gonadotropin hormones from the pituitary with the ovarian steroids as the afferent arm. More recent studies have begun to elaborate a complex series of paracrine processes that operate within the tissues of the ovary and uterus to impose local regulation.
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.