To answer this question, we begin about 1 week after fertilization. So, one week after the female embryo is formed, primordial germ cells start to form that later will become eggs. If you think about it, it’s amazing! The top priority of the human embryo is to make eggs for further procreation and it begins its production before the heart starts beating and before the brain starts forming. In the next 18 weeks or so, those cells will develop into gonads, which in females will contain primary oocytes, immature egg cells. (Gonad, in zoology, is a primary reproductive gland that produces reproductive cells. In males the gonads are called testes; the gonads in females are called ovaries.) By 20 weeks gestation, this process is complete and stops forever. At this point, approximately 6,000,000 oocytes have been formed. The total number of oocytes declines from that point forward. Between 20 weeks and birth, 4,000,000 of those 6,000,000 eggs undergo programmed cell death. Leaving about 2,000,000 oocytes present at birth. From birth until the first menses another 1,500,000 of those 2,000,000 eggs disappear. This leaves roughly 500,000 eggs available for an average reproductive lifetime. The reproductive lifetime is approximately 500 months long (from menarche, the first menstrual period, at 10-12 years to menopause at 50-52 years).
Every month, one egg is ovulated. If there are 500,000 eggs available for 500 months AND each month, we ovulate one egg THEN you can think of there being 1,000 eggs available for each month of woman’s reproductive lifetime! However, this is not exactly accurate because we use many more than 1,000 eggs per month in our early reproductive years, and we have many less than 1,000 per month at the end of our reproductive lifetime.
Each month there is a “pool” of oocytes available. From this pool, the body naturally selects one oocyte for ovulation. Interestingly, there is a direct correlation between the size of the pool and the chance that any one egg in that pool will go on to become a baby. This is reflected in the age-related decline of fertility. When we’re younger, there is a much larger pool from which the egg comes from; therefore, it is much easier to become pregnant. As we get older, the pool, from which the egg comes from, becomes much smaller; therefore, it is much harder to get pregnant.
How Does It Work?
Eggs are microscopic structures that are invisible to the naked eye or ultrasound. They are independent of any hormonal stimulation. Each egg (ovum) sits inside a much larger structure, that is called a follicle. All follicles are distributed throughout the inner wall of the ovaries. Once follicles start to mature (grow) they become visible to ultrasound.
Ovarian Follicle Development
From puberty through the reproductive years, cohort of 10 to 20 follicles are activated monthly by FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone), produced by the Pituitary Gland in the brain. Where are all oocytes that spontaneously activate prior to puberty are destined to degenerate and die, those that are activated at the start of the menstrual cycle have the potential to develop into a mature, fertilizable egg. This is a consequence of the hormonal environment of the adult female. Typically, only one follicle out of each monthly cohort will achieve this state of maturation.
In natural ovulation, there is pulsatile release of GnRH from the hypothalamus that causes timed release of FSH and LH from the pituitary gland. FSH causes the granulosa cells in the ovarian follicles to produce estradiol, which (with inhibin) decreases pituitary FSH secretion through negative feedback.
As I mentioned above, there is a large pool of follicles that continuously diminishes over the time. As it diminishes, 10-20 of those follicles “get rescued” from the programmed cell death, apoptosis, a month and others just die. Out of those 10-20, only ONE follicle with sufficient FSH receptors is able to continue growing despite the decrease of FSH and other follicles will stop growing. When that follicle gets big enough, it sends a signal back to the ovary saying that it’s a dominant follicle and it’s ready and then the ovulation follows.
Once the ovulation occurs, that one egg is released from the ovary and finds its way to the Fallopian tube. If the intercourse is properly timed, sperm finds the way into the Fallopian tube and meets the egg in the Ampulla. The Ampulla of the Fallopian tube is the only place where fertilization can occur. If the sperm and egg meet some other place, fertilization will not occur.
That’s why when you ovulate and have sex too late, you might not get fertilization because the egg might make its way too far down the Fallopian tube before the sperm can get to it. Then the fertilized egg travels down the Fallopian tube for the next 5 or so days when it eventually gets into the uterus and implants into the wall of the uterus. Then 9 months later, the baby is born!