During prenatal maturation, the ova are formed in the ovary from cells called oogonia which proliferate by mitotic division. All of the oogonia enlarge to form primary oocytes, of which about 2 million are present at birth. No primary oocytes form after birth (in contrast to the continuous production of primary spermatocytes in the male after puberty). Primary oocytes begin the first meiotic division before birth but do not complete prophase until after puberty.

During postnatal maturation, the primary oocytes stay dormant in the ovaries until puberty. The primary oocyte increases in size and a membrane, the zona pellucida, forms around it as the follicle matures. Just before ovulation, the primary oocyte completes the first meiotic division, but unlike its male counterpart, the division of cytoplasm is unequal.

At ovulation, the nucleus of the secondary oocyte begins the second meiotic division progressing only to metaphase, then division arrests. If fertilization occurs, the second meiotic division is completed, and the mature oocyte retains most of the cytoplasm, whereas the second polar body is small & degenerates. Only about 200-400 of primary oocytes ever reach full maturity after puberty and are expelled at ovulation during the female's reproductive life.

Every menstrual cycle corresponds to the maturation of an oocyte, which becomes an ovum through division, yielding cells of unequal size (oocytes and polar bodies). There are 2 kinds of normal sperm, with respect to sex chromosomes: 23,X and 23,Y; there is but 1 kind of normal oocyte: 23,X (refers to 23 chromosomes in the complement, made up of 22 autosomes and 1 sex chromosome, X or Y).