How Human Reproduction Works

The two ovaries are the major female sex organs, the counterpart of the male testes. The ovaries make the eggs, or oocytes, which are the female gametes, and produce estrogen, the female sex hormone. Estrogen causes female secondary sexual characteristics such as pubic hair, breast development, widening of the pelvis and deposition of body fat in hips and thighs. The ovaries are located in the abdomen.

Eggs develop inside the ovary and are released upon ovulation into the Fallopian tube, which is lined with fingerlike projections that push the egg through the tube. During the egg's travels through the Fallopian tube, fertilization can take place if sperm is present, and then the fertilized egg will continue to a muscular chamber called the uterus.

The uterus, or womb, is where a fetus develops. It's made of smooth muscle and is normally about the size and shape of a small pear turned upside down. During pregnancy, it can stretch to about the size of a basketball to hold the developing baby. The base of the uterus (the neck of the pear) is a muscular wall called the cervix. In the cervix is a tiny opening, about the size of a pinhead, called the external os. The external os is filled with a thick plug of protein (mucus) that serves as a barrier to the entrance of the uterus. The cervix leads into a smooth-muscle-walled tube called the vagina, or birth canal.

The vagina connects the uterus to the outside of the body, and its opening is covered by sets of folded skin called labia. The vagina receives the male's penis during sexual intercourse and delivers the baby during childbirth. The vagina is normally narrow, but can stretch during intercourse and childbirth.

Finally, two sets of glands, the greater vestibular gland (Bartholin's gland) and the lesser vestibular gland, are located on either side of the vagina and empty into the labial folds of skin. The secretions from these glands lubricate the labial folds during sexual excitation and intercourse.