The human reproductive system usually involves internal fertilization by sexual intercourse. During this process, the male inserts his erect penis into the female's vaginaand ejaculates semen, which contains sperm. A small portion of the sperm pass through the cervix into the uterus, and then into thefallopian tubes for fertilization of the ovum. Only one sperm is required to fertilize the ovum. Upon successful fertilization, the fertilized ovum, or zygote, travels out of the fallopian tube and into the uterus, where it implants in the uterine wall. This marks the beginning of gestation, better known aspregnancy, which continues for nine months as the fetus develops. When the fetus has developed to a certain point, pregnancy is concluded with childbirth or labor. During labor, the muscles of the uterus contract and the cervix dilates over the course of hours, and the baby passes out of the vagina. Human infants are nearly helpless and require high levels of parental care. Infants rely on their caregivers for comfort, cleanliness, and food. Food may be provided by breastfeedingor formula feeding.

in brief