Primary oocytes enter meiosis I and replicate their genomes, but they do not make their first meiotic division. They remain in prophase I until a female begins her first menstrual cycle. Then, each month, one primary oocyte resumes the process of meiotic division.

A primary oocyte becomes a secondary oocyte when the follicle grows and matures and the primary oocyte completes its first meiotic division. Shortly after, the follicle will rupture and release the secondary oocyte to be fertilized. Later, after the oocyte develops into an ovum and is fertilized, it becomes a diploid zygote, which develops into an embryo through the process of mitosis. The ovary is the reproductive organ where the ova are produced, and a gamete is the haploid cell (ovum or sperm) that is fertilized and forms a zygote.

During the menstrual cycle, typically only one follicle matures and is fertilized. The follicles that do not mature are called polar bodies and degenerate. However, sometimes multiple follicles remain and are available to be fertilized, resulting in genetically distinct embryos, known as fraternal twins.  

Typically, each menstrual cycle produces three primary oocytes. Two of these primary oocytes are inevitably smaller than the third and usually disintegrate before maturation and fertilization. These two cells are known as polar bodies. Usually, meiosis results in four daughter cells that are all haploid. However, after meiosis I of oogenesis, there are two daughter cells, one primary oocyte, and a polar body. After meiosis II, the primary oocyte divides into a secondary oocyte and another polar body. However, the polar body that was a result of the first meiotic division usually does not complete meiosis, and thus does not divide again.

The secondary oocyte completes meiosis II only when fertilized by a spermatozoan. After fertilization is initiated, the secondary oocyte begins its second meiotic division, resulting in the formation of a mature ovum and another polar body. At this point, the ovum is ready to fuse with the spermatozoan.

Fertilization typically occurs in the fallopian tube. After fertilization, the diploid zygote will begin the process of implantation into the uterus, where it undergoes mitotic divisions. The resulting multicellular organism is called the embryo, which further develops in the uterus into a fetus.

Normally, one ovum is released every 28 days in human females. These 28 days constitute the menstrual cycle, which may vary in length, but on average, the cycle takes 28 days. A female is born with all the eggs she will ever have, and after the initiation of menstruation, one egg is released every 28 days until menopause.

At the beginning of the menstrual cycle, elevated follicle-stimulating hormone (FSH) stimulates primary oocytes to resume meiosis. A spike in luteinizing hormone (LH) triggers ovulation. Thyroid-stimulating hormone (TSH) and human chorionic gonadotropin (hCG) are involved in thyroid hormone secretion, and in maintenance of the corpus luteum during pregnancy, respectively. Estrogen is a female sex hormone, and has an indirect regulatory role in meiotic divisions of ova.

An oocyte develops in the ovaries during female gametogenesis. If the oocyte eventually becomes an ovum and is fertilized in the fallopian tubes, the resulting gamete will be implanted in the uterus. The placenta is an organ that connects the developing fetus to the uterine wall, and the cervix is the lower portion of the uterus that separates the vagina from the uterus.

When gametes join they form a cell called a zygote. Human sperm and eggs contain 23 chromosomes. Human zygotes contain 46 chromosomes. The type of cell division that produces gametes with half the normal chromosomes is called meiosis.