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As a woman ages, her fertility declines. The eggs inside her ovaries become increasingly unable to develop into a baby, which leads to infertility.
To get pregnant, it is not as much the total number of eggs that matters, but also the proportion of eggs that are of high-enough quality. Eggs must respond to hormones that trigger ovulation, they need to successfully fertilize, and they must kick-start the cell divisions needed to form an embryo. Not all of the eggs in a woman’s ovaries are capable of this, but having an ample supply of good ones means a woman has an adequate “ovarian reserve,” improving her chances of a pregnancy. When the opposite is true, or a high percentage of eggs are of poor quality, the likelihood of infertility increases.
According to Dr. Jonathan Tilly, director of the Vincent Center for Reproductive Biology at Massachusetts General Hospital in Boston, many patients considering fertility treatments fail to realize the strong link between good egg quality and the risk of miscarriage or birth defects. “Yet they are all tied together,” he explains. “It’s like a house of cards—when egg quality is poor, it sets the stage for all these unfortunate outcomes that impair fertility,” Dr. Tilly says. These problems are compounded by the modern fertility trend of women postponing pregnancy to later in life. “First birth rates for women 35 to 44 years of age in the United States have increased by more than eight-fold the past four decades,” Dr. Tilly adds.
Low-quality eggs have internal defects -- problems with their chromosomes or with their energy production -- that prevent them from producing healthy embryos. Women struggling to conceive may not even realize their egg quality is impaired if their periods seem normal.
A woman is born with about one million eggs, but by the time she has her first period the numbers have fallen to less than 400,000. These remaining eggs lie dormant inside a woman’s ovaries, waiting to become activated by hormones, during the monthly cycle (if the woman is going through in vitro fertilization, or IVF, the eggs become activated by hormone treatments used to induce ovulation).
As a woman ages, her eggs accumulate defects. One common issue is that chromosomes within eggs do not move properly as an egg matures and the embryonic cells divide, leading to defective embryos. This explains the higher incidence of miscarriage and birth defects in pregnancies of older women. The odds of having a baby with Down syndrome are over one in 1,000 for women in their 20s, but increases to less than one in 100 among women in their 40s.
By the time women enter their 40s, most of their eggs will be of poor quality. This is reflected in lower IVF success rates as they age. IVF success rates for women are around 45% at age 25 and drop to 35% by their mid 30s. IVF success rates decline even further to 19% by age 40 and 3% at age 44. In contrast, when women over 40 use high-quality eggs donated by younger women during IVF, their live birth rates stay in the 50% range .
This does not mean a woman in her 40s cannot get pregnant; it is just that her odds are severely diminished. By the same token, there are younger women who, due to medical conditions, have a poor ovarian reserve. Surviving cancer patients and women undergoing premature menopause are two examples.
Is there anything a woman can do to maintain or boost the quality of her eggs? Unfortunately, at the moment the answer is no. Women whose egg quality is compromised sometimes consider using donor eggs during IVF. In the future, options may become available to boost the quality of a woman’s eggs, leading to increased chances of a pregnancy.
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