The human reproductive system usually involves
internal fertilization by
sexual intercourse. During this process, the male inserts his
erect penis into the
female's vagina and
ejaculates semen, which contains
sperm. A small portion of the sperm pass through the cervix into the uterus, and then into the
fallopian 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 as
pregnancy, 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
breastfeeding or
formula feeding.