Menstrual cycle is the cycle of changes that occurs in the uterus and ovary for the purpose of sexual reproduction. It is essential for the production of eggs and for the preparation of the uterus for pregnancy. The menstrual cycle occurs only in fertile female humans and other female primates.
In humans, the length of a menstrual cycle varies greatly among women (ranging from 25 to 35 days), with 28 days designated as the average length. Each cycle can be divided into three phases based on events in the ovary (ovarian cycle) or in the uterus (uterine cycle). The ovarian cycle consists of the follicular phase, ovulation, and luteal phase whereas the uterine cycle is divided into menstruation, proliferative phase, and secretory phase. Both cycles are controlled by the endocrine system and the normal hormonal changes that occur can be interfered with using hormonal contraception to prevent reproduction.
Human reproduction is any form of sexual reproduction resulting in the conception of a child, typically involving sexual intercourse between a man and a woman. During sexual intercourse, the interaction between the male and female reproductive systems results in fertilization of the woman's ovum by the man's sperm, which after a gestation period is followed by childbirth. The fertilization of the ovum may nowadays be achieved by artificial insemination methods, which do not involve sexual intercourse.
The reproductive system or genital system is a system of sex organs within an organism which work together for the purpose of reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system.[dead link] Unlike most organ systems, the sexes of differentiated species often have significant differences. These differences allow for a combination of genetic material between two individuals, which allows for the possibility of greater genetic fitness of the offspring.
Menstrual synchrony is a hypothesized process in which women who begin living together in close proximity experience their menstrual cycle onsets (i.e., the onset of menstruation or menses) becoming closer together in time than previously. "For example, the distribution of onsets of seven female lifeguards was scattered at the beginning of the summer, but after 3 months spent together, the onset of all seven cycles fell within a 4-day period."
Martha McClintock's 1971 paper in Nature (journal) says that menstrual cycle synchronization happens when the menstrual cycle onsets of two women or more women become closer together in time than they were several months earlier. Several mechanisms have been hypothesized to cause synchronization.
A social issue (also called a social problem or a social situation) is an issue that relates to society's perception of a person's personal lives. Different cultures have different perceptions and what may be "normal" behavior in one society may be a significant social issue in another society. Social issues are distinguished from economic issues. Some issues have both social and economic aspects, such as immigration. There are also issues that don't fall into either category, such as wars.
Thomas Paine, in Rights of Man and Common Sense, addresses man's duty to "allow the same rights to others as we allow ourselves". The failure to do so causes the birth of a social issue.