What is infertility and how common is it?
Infertility is the reduced capacity to conceive compared to the general population.
- At least 6 million American couples (approximately 10% of the reproductive-age population) have difficulty conceiving.
- 25% of women will experience an episode of infertility during their reproductive life.
- 5-10% of normal, fertile couples take more than 1-2 years to conceive.
Couples are encouraged to seek medical assistance for infertility if:
- The wife is 35 or younger, but fails to conceive after 1 year of regular unprotected intercourse; or
- The wife is older than 35 and fails to conceive after 6 months of regular unprotected intercourse; or
- The patient has had more than 2 spontaneous abortions or stillbirths
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Understanding the Reproductive Process
For pregnancy to occur naturally there must be:
- An adequate amount of healthy sperm
- Open fallopian tubes
- Healthy egg cells
- Normal uterus
The woman’s egg cells are the most important factor because while all the other elements can be ‘fixed’, the reproductive process, natural or assisted, has to start with the woman’s eggs as they are.
The Menstrual Cycle and its Role in Reproduction
The following happens in a natural menstrual cycle:
Each month, the ovaries ‘produce’ approximately 10-20 egg-bearing follicles, out of which one follicle will become dominant and will ovulate the egg destined for ovulation during the cycle.
- These follicles come from the vast follicle pool that a woman is born with, but she will ovulate only about 300-400 eggs during her reproductive life.
- Starting at birth, the follicle pool is continually depleted by natural atresia, which is a programmed degeneration in which the degenerated follicles are simply absorbed into the regular ovarian tissue.
- Likewise, the follicles recruited for possible ovulation each cycle will go into atresia.
- Follicle depletion by atresia explains why, as a woman ages, she will have progressively fewer follicles left (this is called her ovarian reserve).
- The best follicles tend to be ovulated first because they respond best to the reproductive hormones. Therefore, increasing age not only means fewer follicles but also generally poorer quality follicles.
The ovulation process is regulated by two hormones produced in the pituitary gland of the brain:
FSH (follicle-stimulating hormone) secreted after ovulation in the previous cycle recruits the follicles for possible ovulation in the next cycle.
- In the first half of the cycle, it will promote growth and development of the recruited follicles.
- The follicle that best utilizes FSH will become the dominant follicle and outgrow the rest.
- The egg destined for ovulation during the cycle will come from the dominant follicle.
- In response to FSH, the nutrient cells surrounding the egg will produce increasing amounts of estradiol (E2), the dominant estrogen in non-pregnant women, causing both the follicle and the egg to grow.
LH (luteinizing hormone) takes over follicle control towards mid-cycle because it causes the follicle and the egg it contains to mature before ovulation.
- In response to the increasing estrogen, LH secretion from the pituitary will surge. When it does, ovulation will occur in the next 24-36 hours. It is this LH surge that is detected in home ovulation kits.
- A mature follicle measures 18-20 mm, and the estradiol level corresponding to a mature egg is about 300 pg/ml.
Ovulation occurs when the follicle bursts open to release the egg, which, in normal women, is captured by the fingerlike opening of the fallopian tube.
- The egg will reside in the tube and will be fertilizable for at last 36 hours.
- When semen is deposited in the vagina at intercourse, the sperm cells travel through the cervix into the uterus. About 200 sperm cells will reach the fallopian tubes from the uterus.
- Fertilization occurs when one sperm cell penetrates the waiting egg.
After ovulation, the follicle that ovulated turns into a corpus luteum.
- In continuing response to LH, the primary function of the follicular cells this time is to produce progesterone, the hormone that maintains the pregnancy if it occurs. It also continues to produce a small amount of estrogen.
- Progesterone and estrogen acting together cause the lining of the uterus (endometrium) to thicken as it adds cell layers appropriate for the reception of an embryo if fertilization has occurred.
- A fertilized egg usually stays in the fallopian tube for 3-5 days by which time it will have become an embryo, and then it travels to the uterus where it will seek to implant itself in the uterine lining.
- If it successfully implants, then pregnancy occurs. The embryonic cells will attach to the lining and start interacting with the endometrial cells to form the placenta, which will be the lifeline carrying blood and nutrients to the embryo (and eventual fetus) from the mother.
- If no embryo is present or if an embryo fails to implant, then the uterine lining will start to break down about seven days after ovulation and will be shed as menstrual blood in the next cycle. Menstrual blood consists of endometrial cells, blood, and other endometrial secretions.
- Progesterone levels will drop, and the hormonal levels of FSH, LH and estradiol will return to cycle baseline levels.
- Fourteen days after ovulation if no pregnancy has occurred, the new menstrual cycle begins.