Today it is more common than ever for women to delay child-bearing for various reasons. Twenty percent of women have their first child after the age of 35 in the US. Pursuing a career, attaining financial stability and finding a suitable partner are common reasons cited. The oldest woman documented to give birth with the use of her own eggs was 56 years old, but most women will have great difficulty conceiving after age 42. Many scientific studies have shown that a woman’s fertility (the chance of conceiving in a given month) declines a small amount after the age of 30. The odds of conceiving decline even more rapidly after the age of 40. One way of measuring the fertility of different age groups is to review the pregnancy success rates of women undergoing the form of assisted reproductive technology known as In-Vitro Fertilization (IVF). IVF can overcome various causes of infertility such as low sperm counts or blocked fallopian tubes. Thus, looking at the IVF success rate removes some of the confounding variables that affect fertility. The Center for Disease Control (CDC) keeps careful records of the pregnancy success rates of all IVF centers. The success rates are reported according to the woman’s age. In the most recent national report, the chances of IVF resulting in a live birth were as follows:
Less than 35 years 37%
35-37 years 30%
38-40 years 20%
41-42 years 11%
Data is shown below in a graph from the CDC.
For a woman wanting to delay child-bearing, it seems unfair that her fertility declines with age even though a man’s fertility does not decline significantly. Why the difference between the sexes? The biological answer is that men continue to make new sperm cells every day of their adult lives. Women, on the other hand, are born with all the eggs they will ever have. New ones cannot be made. Thus a 40 year old woman has eggs that have been around for 40 years. They’ve been exposed to various toxins and cosmic radiation over the years. Contrast this to a man’s sperm which is only a few days or weeks old. In addition to the problem with the aging eggs, a women has to contend with the fact that her supply of eggs is limited and will eventually be exhausted. Menopause marks the time when no more usable eggs remain. The average age of menopause is 50. A woman has approximately 300,000 immature eggs in her body at the time of puberty. So what becomes of those eggs between puberty and menopause? Were they all released during the once-a-month ovulation process? No. Usually only one egg will reach the fully mature state necessary to ovulate during each menstrual cycle. The average woman has 450 menstrual cycles during her lifetime. The other 299,550 or so eggs slowly degenerate over time at a rate of about 25 per day. The rate of loss accelerates after the age of 37. Fertility medications will not alter this rate of egg degeneration. Some things are known to speed up the loss of eggs. An example is certain chemotherapy agents used to fight cancer. Smoking cigarettes also appears to hasten the depletion of usable eggs. In fact menopause typically arrives one or two years earlier in women who smoke. A family history of early menopause is also a significant factor.
Many women have fears that their chances of having a baby with Down’s Syndrome or other chromosomal abnormalities is outrageously high; however, this is not the case. At age 40, the chance of having a pregnancy with Down’s Syndrome is fairly low – only 3%. One risk, however, that is increased is the chance of miscarriage. On average 40% of the pregnancies in women over the age of 39 will spontaneously abort. The most common reason for these miscarriages is a chromosomal imbalance that is not compatible with further fetal development.
Just because a woman is over 40 years of age does not mean she needs to give up all hope of having a baby. There is a simple blood test that her gynecologist or fertility specialist can order to help determine if she has an adequate reserve of eggs to allow for a reasonable chance of becoming pregnant. This blood test is drawn on the 2nd or 3rd day of the woman’s menstrual cycle. This test for ovarian reserve consists of both an FSH level and an estradiol level. FSH represents Follicle Stimulating Hormone, made by the woman’s pituitary gland at the base of the brain. Its purpose is to stimulate maturation of the egg-containing follicles that remain in the ovaries. If the number of good quality follicles is dwindling down, more and more FSH is needed to mature an egg that cycle. Once a follicle begins to mature (typically day 5-10), the FSH level drops since the pituitary has fulfilled its duty that month. If no pregnancy arises, then the process begins again with the next menses. An FSH level under 10 is considered normal. A level of 10-15 is concerning. It indicates the early stages of diminished egg reserve. An FSH of 15-20 indicates a definite low supply of eggs. Normal pregnancies in women with an FSH over 20 are quite rare except in woman under the age of 30. The FSH level typically varies from month to month. A woman’s chance for normal conception is diminished even if she has only one FSH test that is abnormal and subsequent FSH levels under 10. That is, her pregnancy prognosis is determined by the level of her highest-ever FSH test. The other component of the ovarian reserve blood test mentioned is the estradiol level. A maturing follicle produces estradiol. When the blood test is drawn early in the menstrual cycle, the estradiol level should be less than 50. If the estradiol level is greater than 50 then the test that month is not fully accurate because the high estradiol is suppressing the FSH level below what it would have been before follicular maturation. Some months a woman may begin the egg maturation process earlier than usual. The test is simply repeated with the next menses to try again to assess the woman’s egg reserve.
This graph shows the effects of age and FSH levels on the success rates for IVF.
A woman who has a diminished number of remaining eggs has a lower chance of conceiving a pregnancy with her own eggs, even through the use of fertility treatments. This is true even when the woman is still having regular menstrual cycles. In these cases the eggs that remain are less likely to fertilize and the resulting embryo is less likely to continue to grow. Why? It is hypothesized that the remaining eggs are less optimal because the best ones have already been selected earlier.
If possible, a woman who wishes to have a child should try to conceive before her mid- to late- 30′s. Sometimes a woman will begin running low on usable eggs even in her early -30′s. Fortunately there is one successful treatment for women who lack a sufficient egg reserve. The use of donated eggs allows these women to carry a pregnancy that is genetically related to her husband even though it is not her genetic offspring. Such a donor egg cycle uses the technique of IVF with the eggs of a willing donor. The oldest woman to deliver a baby through the donor egg technique was reported in Italy. She was 63 years old! This technology appears to allow women to enjoy reproductive flexibility similar to what men have with regard to delaying child-bearing, except that the baby does not carry the genetic make-up of the birth mother.