June 16, 2004 -- A new study from the U.K. suggests that the size of a woman's ovaries may be a better predictor than age of when her biological clock will stop running, but experts contacted by WebMD remain skeptical.
The U.K. researchers devised a mathematical model showing a direct relationship between ovarian size, as measured by ultrasound, and the number of eggs remaining in the ovaries of women during their childbearing and premenopausal years.
They say the findings could lead to a better way of assessing whether a woman is a good candidate for fertility treatments such as in vitro fertilization that require retrieval of a woman's eggs for fertilization. The findings may also help women more accurately determine how long they have to start a family.
"In essence, it means we now have the potential to be able to tell a woman how fast her biological clock is ticking and how much time she has before it will run down," says lead researcher Hamish Wallace, MD, of the Royal Hospital for Sick Children in Edinburgh, Scotland.
Quantity vs. Quality
Women's ovaries get smaller throughout their childbearing years, and earlier research has linked this to a decline in available eggs. In their report, the researchers theorize that ovary size measured by volume can serve as an accurate measure of future fertility, independent of age.
The results of their finding are published in the June issue of the journal Human Reproduction.
But experts say even if ovary size accurately predicts the number of eggs a woman has left, it still only tells half the story.
"A woman's [fertility] is determined not only by the number of eggs left in the ovary, but also by the quality of those eggs," infertility expert and medical director of the WebMD Fertility Center Amos Grunebaum, MD, tells WebMD.
"There is still a lot we don't know about why certain eggs get fertilized and implant. Some infertile women have plenty of eggs, but they may not be good enough to achieve pregnancy."
Owen Davis, MD, president of the Society for Assisted Reproduction Technology, calls the report "intriguing" but says more studies are needed to determine if the model has value in real-life patient care.
"The findings are purely theoretical at this point," he tells WebMD. "It is not clear if it is possible to use ovarian volume to determine the number of years a woman has before she undergoes menopause."
Researchers believe a woman is born with a finite number of eggs and their numbers decline with age. While the average age that most women reach menopause is around 50, fertility starts to decline roughly one to two decades prior to that. A small percentage, however, remain fertile well into their 40s.
Wallace and mathematician Thomas Kelsey, PhD, developed their model to estimate what they refer to as a woman's "reproductive age." Kelsey tells WebMD they are currently planning studies to test its usefulness.
"Most (assisted reproduction) centers don't treat women who are much older than 40, but some women may have many more years of fertility," says Kelsey. "There may be many good reasons for turning a particular woman away at 40, but age shouldn't be the only one."
Age and a blood test measuring levels of a hormone that stimulates egg growth and maturation, known as follicle stimulating hormone (FSH), are commonly used to determine a woman's chance of conceiving.
Even if studies prove that ovarian size is a clinically useful measure of fertility, Davis says age will remain the best predictor for most women. Davis is associate director of the IVF program at Weill Medical College of Cornell University's Center for Reproductive Medicine and Infertility.
"We know that a woman's chance of getting pregnant is very, very poor at 45, regardless of her FSH," he says. "The same is probably true for ovarian volume."
Grunebaum, who is director of clinical maternal-fetal medicine at Weill Medical College, agrees.
"Until I see studies to confirm this association, I would never tell a woman who is 42 that she has plenty of time to have a baby on the basis of ovary size alone," he says.