The number of genera are far fewer; for example, there are a number of species of Polystichum (one of which is the sword fern), and many of them look so similar to the casual observer that it would seem that there are fewer native ferns than is actually the case.
The egg is released around day fourteen in the cycle and the thickened lining of the uterus is to provide nutrients for a potential developing baby. If pregnancy does not occur, the lining is released in what is known as menstruation. A number of problems with menstruation may occur. A lack of periods, known as amenorrhea, is when periods do not occur by age 15 or have not occurred in 90 days. Periods also stop during pregnancy and typically do not resume during the initial months of breastfeeding.
The ovulatory phase begins with a surge in luteinizing hormone and follicle-stimulating hormone levels. Luteinizing hormone stimulates egg release (ovulation), which usually occurs 16 to 32 hours after the surge begins. The estrogen level peaks during the surge, and the progesterone level starts to increase.he cycle ends just before the next menstrual period. Menstrual cycles normally range from about 25 to 36 days. Only 10 to 15% of women have cycles that are exactly 28 days.