Development of the female reproductive system begins when the individual is a fetus. Development of the eggs, or ova (singular, ovum), is called oogenesis. Oogenesis begins when the developing female fetus is between 8 and 20 weeks of gestation, or pregnancy. At this stage the fetus contains many oogonia, which are cells that eventually mature into viable ova. An oogonium is diploid (2n2c), meaning it has two full sets of chromosomes (46) and every chromosome pair has two chromatids.
The first stage of oogenesis is oocytogenesis. In this process, a diploid oogonium (2n2c) undergoes meiosis I to form a diploid primary oocyte (2n4c). An oocyte is a cell that will give rise to a mature ovum and three polar bodies via meiosis. It is precisely the duplication of DNA and initiation of meiosis that differentiates oogonia from oocytes. Oocytogenesis is complete by about the third trimester of fetal development. The diploid primary oocytes are halted in prophase I of meiosis I. The "4c" of 2n4c means that the primary oocyte has four sister chromatids for every chromosome pair. The primary oocytes will remain arrested in prophase I of meiosis I until events of the ovarian cycle begin sometime after puberty. Therefore, it is believed that a woman is born with all the oocytes she will ever have.During a woman's ovarian cycle, one primary oocyte within a follicle begins the maturation process. Though rare, there are occasions when more than one primary oocyte begins to mature (this leads to fraternal twins or triplets if the eggs are fertilized). The primary oocyte resumes meiosis I, in this case known as ootidogenesis. This produces one haploid secondary oocyte (1n2c) and one polar body. The "1n" indicates that the secondary oocyte has one full set of chromosomes, or 23 chromosomes. The "2c" means each chromosome has sister chromatids. A polar body is a haploid (1n) cell produced during meiotic divisions of an oocyte that cannot be fertilized. The polar body usually disintegrates and the cellular materials are reabsorbed by the woman's body. The secondary oocyte is halted in metaphase II of meiosis II unless fertilization occurs. Another term for a secondary oocyte is a gamete or germ cell. A gamete is a haploid cell that can unite with another haploid germ cell to form a diploid zygote. If fertilization does not occur, the secondary oocyte degenerates. If fertilization does occur, meiosis resumes, resulting in an ootid (1n1c), a cell that is an immediate precursor to a mature ovum, and one more polar body. The ootid differentiates into an ovum (1n1c), allowing for the union of sperm and egg to form a diploid (2n2c) zygote.