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.
The estrous cycle (also oestrous cycle; derived from Latin oestrus and originally from Greek οἶστρος meaning sexual desire) comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian therian females. Estrous cycles start after puberty in sexually mature females and are interrupted by anestrous phases or pregnancies. Typically, estrous cycles continue until death. Some animals may display bloody vaginal discharge, often mistaken for menstruation, also called a "period".
Fertility testing is the process by which fertility is assessed, both generally and also to find the fertile window. General health affects fertility, and STI testing is an important related field.