The Egg’s Journey
We live in a culture where sex is everywhere: talk shows, magazine articles, advertising, and movies—just to name a few. But when it comes to sex for reproduction, not many people appreciate how the sperm and egg come together and how very delicate and sensitive the process is.
Every woman is born with immature eggs in her ovaries. Just before birth, the number of eggs in the baby girl’s ovaries is roughly a few million, but by the time puberty begins, this number has diminished to about 400,000. Of these, only three hundred or so will ever be released. As you age, the number of eggs in your ovaries continues to diminish until the ovary runs out of eggs and you enter menopause (somewhere between 45 and 55 years old, usually).
The development and release of the egg depend on a delicate balance of hormones, chemicals that signal the body’s organs to do particular jobs. Some of these hormones are produced in the ovaries. Others come from two glands in the brain, the hypothalamus and the pituitary.
The process of preparing an egg for release begins the first day of menstruation. During menses, the level of the hormone estradiol is at its lowest and this causes the hypothalamus and pituitary to release follicle stimulating hormone (FSH) and luteinizing hormone (LH). These two hormones act in concert to recruit about 20 eggs and to stimulate the ovary to produce estradiol. Estradiol encourages the growth of the lining of the uterus so that it becomes thick and lush.
Although approximately 20 eggs may start this process each month, only one egg usually survives. The mature egg(s) is released in response to a sudden release of a large amount of LH (this is what is detected with ovulation predictor kits), which occurs approximately 14 days after the beginning of a menstrual period. The LH surge signals the ovary to begin producing progesterone, which will prepare the lining of the uterus for implantation of a fertilized egg. Once released from the ovary, the egg begins its journey by entering the fallopian tube where it may encounter eagerly awaiting sperm and become fertilized.
The fertilized egg continues making its way through the fallopian tube during which time it begins to divide, resulting in the formation of a blastocyst. Six to 10 days (cycle day 20 to 24) after ovulation/fertilization, the blastocyst arrives in the uterine cavity and implants in the endometrium, or uterine lining. If implantation occurs, the blastocyst produces human chorionic gonadotropin (hCG), which prolongs the secretion of estrogen and progesterone from the ovary until the placenta is developed sufficiently to take over this function. If fertilization or implantation fails to take place, the ovary “automatically” stops production of estrogen and progesterone, causing the prepared endometrium to be cast off as menstrual flow.