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.
It's perfectly normal to notice some clumps from time to time during your period. These are blood clots that may contain tissue. As the uterus sheds its lining, this tissue leaves the body as a natural part of the menstrual cycle. So clots of tissue are usually nothing to be concerned about.