Obstetrics - The subdiscipline of medicine that deals with pregnancy and childbirth and the period immediately after birth oestradiol the major female sex hormone. It is produced by the OVARY and is responsible for development of the breasts, sexual characteristics and premenstrual uterine changes.

Oestrogen - One of a group of steroid hormones secreted mainly by the ovaries and, to a lesser extent, by the adrenal cortex and placenta. (The testicles also produce small amounts.) Oestrogens control the female secondary sexual characteristics, i.e. enlargement of the breasts, change in the profile of the pelvic girdle, pubic hair growth and deposition of body fat. High levels are produced at ovulation and, with progesterone, they regulate the female reproductive cycle. Naturally occurring oestrogens include oestradiol, oestriol and oestrone. Synthetic varieties are used in the contraceptive pill and to treat gynaecological disorders.

Oncogene - A gene that has the ability to induce a cell to become malignant. In addition to genes that induce tumor formation, there are antioncogenes that suppress tumors.

Oncologist - A physician who specializes in treating cancer. (There are medical, surgical, and radiation oncologists.)

Oncology - The study and treatment of cancer.

One-Step Procedure - A procedure in which a surgical biopsy is performed under general anesthesia. If cancer is found, a mastectomy or lumpectomy is done immediately as part of the same operation. This is rarely done today.

Oophorectomy - Surgical removal of the ovaries, sometimes performed as a part of hormone therapy

Orange Peel Skin - Skin that has the appearance of the peeling of an orange. The pitting appearance is caused from inflammation and edema of the breast caused by benign or cancerous diseases.

Orchidectomy - Removal of one or both testicles (castration), usually to treat a malignant growth.

Orchidopexy - The operation performed to bring an undescended testicle into the scrotum. It is undertaken well before puberty to ensure subsequent normal development.

Orgasm - The climax of sexual arousal which, in men, coincides with ejaculation and comprises a series of involuntary muscle contractions. In women, there are irregular contractions of the vaginal walls.

Osteoporosis - Softening of bones that occurs with age, calcium loss, and hormone depletion. After menopause, women are particularly suceptible to this condition without some form of hormone supplement.

Ovarian cyst - A sac filled with fluid that develops in the ovary. Most are benign but their size may cause swelling and pressure on other organs. For those cysts that do become malignant, it is possible that its discovery comes too late to allow successful treatment. Ultrasound scanning can be adopted to detect tumours at an early stage.

Ovariotomy - Literally cutting into an ovary, but more generally used for surgical removal of an ovary or an ovarian turnout.

Ovary - The reproductive organ of females, which produces eggs (ova) and hormones (mainly oestrogen and progesterone). There are two ovaries, each the size of an almond, on either side of the uterus, and each contains numerous Graafian follicles in which the eggs develop. At ovulation an egg is released from a follicle. The follicles secrete oestrogen and progesterone, which regulate the menstrual cycle and the uterus during pregnancy.

Ovulation - The release of an egg from an ovary (i.e. from a mature Graafian follicle), which then moves down the Fallopian tube to the uterus. Ovulation is brought about by secretion of luteinizing hormone secreted by the anterior pituitary gland.

Ovum - The mature, unfertilized female reproductive cell, which is roughly spherical with an outer membrane and a single nucleus.

Oxidative burst - The rapid combination of a substance with oxygen.