The menstrual cycle occurs in the uterus and ovary and makes sexual reproduction possible. The menstrual cycle occurs monthly and occurs so that the ovary can produce eggs and the uterus can prepare for the fertilization of an egg. (Wikipedia) The menstrual cycle is a complex cycle and is controlled by hormones produced by many different glands. The hypothalamus causes the pituitary gland to produce chemicals that then cause the ovaries to produce the sex hormones estrogen and progesterone. Each gland and structure is influenced by the activity of another which is biofeedback. It is assumed that the menstrual cycle lasts approximately 28 days and occurs in four phases: the menstrual phase (days 1-5), the follicular phase (days 1-13), the ovulation phase (day 14) and the luteal phase ( days 15-13). 28). The counting of days of the menstrual cycle begins on the first day blood comes out of the vagina. (Women's Health) Puberty is the time in a person's life when the body begins to mature sexually; the hormone estradiol is produced by females. Puberty begins with hormonal signals going to the brain and then to the gonads, the ovaries in the female. (NHS Choices Puberty) A hormone is one produced by the gonads that stimulates the growth, function and transformation of the brain, skin, hormones, breasts and sexual organs. During puberty it causes different changes, the physical changes are breast development, pubic hair, body shape (hips widen), vagina and uterus, menstruation and body odor. (Wikipedia) The first phase, the menstrual phase, also known as the menstrual cycle, begins on the first day of menstruation and lasts five days. During this stage the uterus sheds its inner lining of blood vessels and soft tissue which leaves… middle of paper… heron, estrogen and inhibin, to decrease so the uterus can prepare to shed blood vessels. tissues produced for the fertilized egg. (Wikipedia)A fertilized egg cell, the zygote, is produced when two gametic cells, male and female gametes, fuse through sexual reproduction. After sexual reproduction, a single sperm can live between 48 and 72 hours. The sperm cells begin swimming towards the fallopian tubes to fertilize the egg cell waiting in the fallopian tubes. Fertilization lasts about 24 hours; once the sperm enters the egg, it leaves plasma behind, and the egg cell changes its surface so that another sperm cannot penetrate the egg cell. After the sperm enters the egg, it completes its second meiotic division, which forms an ovum and a second polar body. The first signs of pregnancy can be detected within 24-48 hours after fertilization. (Cronolab)