After puberty, female fertility increases until the 20s, then decreases after the early 30s, but there are modern methods allowing women to extend their childbearing years.
There is a dramatic drop in fertility beginning around 35, with spontaneous pregnancy being rare in women aged over 44. This is largely due to the fact that a woman’s complement of eggs is fixed before birth. Even before puberty, eggs begin to develop every day and unless ovulated, degenerate and cannot be replaced.
Menopause, the cessation of menstrual periods, is due to the total loss of eggs. It generally occurs in the mid-40s to mid-50s, and marks the absolute cessation of natural fertility.
Understanding the facts of fertility and age is important for women who wish to plan their lives. Two well-conducted studies from France are useful to review. In French women undertaking donor insemination in the early 1980s, the monthly chance of pregnancy was 11 percent in women under 30, 7.6 percent at ages 31 to 35, and 5 percent over 35. In French women taking no fertility therapy in the early 2000s, the monthly fertility was 11 percent at age 30, 9 percent at 35 and 5 percent at 40. The similarity of these results suggests that the results are accurate, and so I have used these numbers for years in counseling my patients.
A 5 percent monthly fertility rate corresponds to 24 percent of women being pregnant within six months. An 11 percent monthly fertility rate corresponds to 51 percent of women being pregnant within six months.
This relationship between age and female fertility is often referred to as the “biological clock,” and it must not be ignored. However, our modern culture encourages delaying childbearing, with the result that many healthy women who were perfectly fertile at 30 will have infertility problems at 35 or 40. This creates the frustrating problem of infertility in women who had every expectation of being able to delay childbearing until a desired time.
Various lifestyles and medical techniques have been suggested to maintain female fertility. Attempts to preserve functional eggs within the ovary by suppressing ovulation with birth control pills or hormone-reducing injections have failed, because the initial development of the egg is spontaneous and not hormonally-induced.
Several successful techniques have been developed, though. From advanced reproductive therapies, we have learned to stimulate the ovaries to make numerous eggs, which can now be harvested with a needle and frozen for future use even without being fertilized. This is an exquisitely fine technique, as the mature human egg is in the process of dividing its DNA, but it has proven successful. Freezing unfertilized eggs permits a woman to preserve her fertility without concern for a particular partner.
Another option, which is more radical but simpler in technique, is to perform a laparoscopy (a “belly button” operation) and to surgically remove strips of normal ovary, which are then frozen. At the desired time, these can be transferred back into the abdomen and used to create a spontaneous pregnancy. This technique, which has the advantage of not requiring a prolonged hormonal treatment, is especially useful for women who are found to have fertility-wasting cancers; to perform a laparoscopy requires only a few days’ delay in their cancer therapy, which is likely innocuous, while allowing the long-term maintenance of fertility, albeit with the necessity of a second procedure after the woman is cancer-free.
Egg number and quality is not the only factor leading to reduced fertility. Older women tend to have older partners, who are likely to have lower sperm counts. The passage of years allows the development of anatomic issues, metabolic problems and chronic disease that impact fertility, including fibroids, endometriosis, anovulation (lack of ovulation in the menstrual cycle), insulin resistance, polycystic ovaries, thyroid disorders and diabetes.
All of these matters can be addressed, but this takes time and effort and is not always successful. A useful guide that I used in my practice for 25 years is to advise women to have their first baby by 35 and their last by 40.
Women who choose to delay having children to after 35 should live fertility-enhancing lives. They should not smoke tobacco or marijuana, should maintain a healthy weight and should exercise regularly. They should also give serious thought to egg preservation, which can extend “normal” fertility until the 50s.
Robert Howe is a reproductive endocrinologist with extensive experience in fertility and infertility. After 29 years of academic and private practice, he joined Cooley Dickinson Hospital this year.