Humans are sexually reproducing organisms with males having a reproductive system that includes a pair of testes, accessory ducts, glands, and external genitalia, and females having a pair of ovaries, oviducts, a uterus, vagina, external genitalia, and mammary glands. The female reproductive cycle is called the menstrual cycle, which starts at puberty and involves cyclical changes in the ovary and uterus induced by pituitary and ovarian hormones. Coitus followed by fertilization at the junction of the isthmus and ampulla leads to the formation of a diploid zygote, and after approximately nine months of pregnancy, the fully developed fetus is ready for delivery.