The female reproductive cycle is complex. Hormone levels are constantly changing as girls become women, and as women ride their cycles through phases every month. Then, when women reach middle-age, they go through even more changes as their hormones take a drastic dive. This marks an end to their fertility.
The egg cell is much larger than any other cell found in the human body. It has a diameter roughly the thickness of a strand of hair, making it thirty times the size of a sperm cell. Unlike sperm, eggs do not have tails and are thus immotile — meaning that they cannot move.
All egg cells are in an immature state until they have gone through a special process. Before an egg cell can be fertilized, it needs to mature in a nest of cells within the ovary called a follicle. During the follicular phase of the menstrual cycle, about 20 follicles start to grow and mature. One of these follicles at some point of time is chosen as a dominant. It then bursts through the follicle in the ovary in a process known as ovulation. The fallopian tube sweeps it in, and it can either be fertilized by present sperm or begin to travel down toward the uterus, unfertilized.
Unlike men who produce sperm constantly, women cannot make eggs. Every woman is born with all the eggs she will ever have. At birth, a baby girl has approximately from 700.000 to 2 million eggs (oocytes). Every single month of her life until she reaches puberty, she loses about 11,000 eggs. By the time she is in her teens, she has between 300,000-400,000 eggs and fewer than 500 will be ovulated.
From puberty on, a woman loses around 1,000 eggs each month, independent of hormone levels, ovulation, or any other fertility factors.
A woman becomes infertile when she has run out of eggs. This usually happens at around the age of 40 and is followed by menopause about ten years later.
You may think that because a woman ovulates and releases one mature egg per month, that she should lose one egg per month. This couldn’t be further from the truth. As mentioned above, fertile women lose on average 1,000 eggs per month. How does that happen?
Every month, 15-20 follicles selected from the pool begin a process of maturation each one hoping to become that one lucky egg chosen for ovulation. The follicle chosen for ovulation is called a dominant follicle and it grows up to approximately 22 millimeters.
There is a test that can be done at any age and any phase during the menstrual cycle that can estimate a woman’s ovarian reserve.
The test measures a woman’s level of anti-Müllerian hormone (AMH), a hormone secreted by follicles. The level of this hormone reflects the number of follicles, thus the average number of eggs a woman has in reserve.
AMH level of 0.5 ng/mL is considered adequate ovarian reserve, whereas levels less than 0.15 ng/mL are suggestive of a reduced follicle pool and decreased pregnancy rates.
Typically, AMH testing is recommended for:
- Women who have been trying to conceive for over year if they are under the age 35 or women who have been trying to conceive for over 6 months if they are over the age of 35.
- Women who are considering IVF (in vitro fertilization) or other fertility treatments. Low AMH levels may predict poor response to IVF.
- Women who have had chemotherapy or ovarian surgery.
- Women with a suspected ovarian tumor.
- Women who wish to conceive in the future and would like to know where they stand.
The most accurate assessment of ovarian reserve includes a combination of AMH testing with an antral follicle count (AFC).
The AFC test uses an ultrasound scan to view the number of small (2-10 mm in diameter) antral follicles present between days 2 and 4 of the menstrual cycle in both ovaries. The presence of 4 to 10 antral follicles is a sign of good ovarian reserve, whereas lower follicle numbers suggest poor reserve.
These tests combined allow your doctor to make an accurate evaluation of your fertility.
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From the time a mature oocyte is released from its follicle, there are only 24 hours in which it can be fertilized. After this time, the woman is not fertile again until the next time she ovulates. The day on which she ovulates, and the five days beforehand are referred to as a woman’s “fertile window.”
The fertile window begins five days before ovulation because sperm can live in a woman’s body for up to 5 days. As long as there is sperm present at the time a woman ovulates, there’s a chance that she can get pregnant. The closer to ovulation, the more fertile a woman becomes.
Average conception rates:
Conceive within 1 month – 20% chance
Conceive within 6 months – 60% chance
Conceive within 9 months – 75% chance
Conceive within 12 months - 80% chance
Conceive within 18 months – 90% chance.
For couples trying to conceive, it is recommended that they have intercourse every two days throughout the fertile window and leading up to ovulation. The big question is how a woman can determine when she will ovulate, and there are a few ways.
Track your cycle
The first step in determining when you will ovulate starts with tracking your cycles. The first day of your period is the first day of a new menstrual cycle. Ovulation takes place 12-14 days before your next period, so in a 28-day cycle, it will happen at around the 14th day of your cycle. Some women have shorter or longer cycles, so it definitely isn’t that way for everyone. The average fertile woman ovulates between days 10-21 of each menstrual cycle.
Once you have determined the average number of days in your cycle, you can narrow it down even further and track ovulation by measuring your basal body temperature (BBT) every day throughout your cycle. Your BBT will show that you have ovulated, not predict when you are about to. This is useful for planning your next cycle, and also just to confirm that you did, in fact, ovulate instead of having an anovulatory cycle.
In order to have success with this method, you need to be consistent. Take your temperature every day first thing in the morning — before you even get out of bed — for several cycles. You should see an increase of 0.72 degrees F or 0.4 degrees C the day after ovulation, that will remain until the start of your next cycle.
Use an ovulation predictor kit
Right before ovulation, there is a surge in the luteinizing hormone (LH). An ovulation predictor kit (OPK) picks up on the increase in LH based on the concentration of LH levels in the urine and indicates that by showing a dark test line next to the control line. The kit looks much like a home pregnancy test. When the test line is darker than the control line, ovulation will most likely occur within the next 12-36 hours.
Ovulation tests are a great way to predict ovulation, however, they are not always 100% accurate. Women with polycystic ovaries may experience a surge in LH that is not followed by ovulation. Occasionally, even young, healthy women with no known fertility problems will have anovulatory cycles.
Check your cervical mucus
Another method for predicting ovulation is by paying close attention to changes in your cervical mucus. The amount and appearance of discharge changes based on how much estrogen a woman’s body is producing. When the cervical mucus increases and becomes clear, slippery, and stretchy — much like egg whites — a woman has reached the most fertile time in her cycle.
From only 9 weeks gestation, a female fetus’ eggs begin to form. By 20 weeks gestation, a female baby’s reproductive system is fully developed, and her ovaries contain 6 to 7 million eggs. This is the most eggs she will ever have.
From 20 weeks until she is born, the fetus will lose the majority of her eggs, and she will be left with around 2 million at the time of birth. By the time she reaches puberty, she will have 300,000-400,000 eggs, only about 350 of which will ever be viable options for fertilization — one every month from puberty until she reaches her menopause.
When thinking about this incredibly complex system, consider that you were the lucky winner — the 1 out of 7 million that made it through maturation, fertilization, gestation, and birth.
If you are trying to conceive, talk to your doctor about doing an AMH test to determine your ovarian reserve. In addition, try the methods listed above to predict your fertile window and increase your chance for conception.