Resetting the Biological Clock

New study shows male, female fertility declines sooner than expected

MONDAY, April 29, 2002 (HealthDayNews) -- Despite the rash of older Hollywood mothers-to-be making headlines, researchers say fertility declines sooner for both sexes than previously thought.

The decline starts as early as the mid-20s for women and the late 30s for men, according to a new report published tomorrow in the European journal, Human Reproduction.

"There was a nearly 50 percent drop in the probability of pregnancy -- given optimally timed intercourse -- between women in their early 20s and women in their late 30s. For men, there was a 40 percent decline in the probability of pregnancy between age 35 and age 40," says David Dunson, study author and a researcher at the National Institute of Environmental Health Sciences in North Carolina.

If this sounds quite a bit different -- not to mention more ominous -- than what you've been reading in popular magazines, that's because this is the first large-scale study of fertility to adjust for differences in frequency and timing of sex, two factors that figure heavily into how quickly or easily a couple conceives.

"This adjustment makes our data uniquely reliable for studying differences in fertility that are driven by biological and not behavioral factors," Dunson says.

The news, however, isn't all bad. The study also found that fertility is, by and large, a very personalized piece of biology. He reports variations in pregnancy rates ranging from as low as 20 percent in any given cycle to as high as 60 percent for couples of the same age, with variances largely dependent on a variety of physiological and environmental factors.

Fertility expert Dr. Jaime Grifo agrees: "Egg quantity and quality is important, but so are other factors such as the quantity and quality of cervical mucous, the condition of a woman's tubes and uterus, the health and mobility of man's sperm. These are all things that can dramatically change pregnancy odds at any age."

"The truth is, the older you are when you try to get pregnant, the harder it is to conceive. But harder doesn't equal impossible -- it just may take longer," says Grifo, director of reproductive endocrinology at New York University's Medical Center and president of the Society for Assisted Reproductive Technology.

That's what Dunson and his colleagues found. They say that while the drop in fertility affects your chances of getting pregnant in a single menstrual cycle, it doesn't impact your overall likelihood of conceiving. You may just have to try a few months longer than you thought.

The other piece of good news: The window of fertile opportunity doesn't change with age. As long as a woman is ovulating, all couples still have pretty much the same time period in which to conceive: six days before ovulation.

Dunson's study relied on data from 782 women between the ages of 18 and 40 from seven European medical centers. All were part of a large, multinational European fertility study.

The women were asked to keep daily records of body temperature (which changes in relation to ovulation and peak fertility times), and to record the days on which they had intercourse and their menstrual cycle started.

Ultimately, the researchers collected data on 5,860 menstrual cycles. Dunson's team then broke down the data into four age groups: 19 to 26, 27 to 29, 30 to 34, and 35 to 39. The researchers also factored out any age differences between women and their partners.

The final result: All totaled, 433 pregnancies occurred. When intercourse took place within two days before ovulation (considered the peak time), and partners were roughly the same age, couples between 19 and 26 had a 50 percent chance of conception during any single menstrual cycle.

Between ages 27 and 34, however, those odds dropped to around 40 percent. By the time a couple reached the 35-to-39 age group, the chance for conception during a single menstrual cycle was less than 30 percent. If the man was five years older, those odds dropped again, to around 20 percent.

If those numbers sound a bit high -- most fertility experts estimate pregnancy rates at between 11 percent and 25 percent during a single cycle -- here's why:

"The 50 percent is in cycles with intercourse timed on the most fertile day of the cycle -- two days prior to ovulation," Dunson says. Most women, he says, do not know when their most fertile day occurs. So, in many instances, the pregnancy probability will typically be below this theoretical maximum.

Dunson also offers this advice: "Most of the fertile interval, including the most fertile day of the cycle, occurs prior to the day of ovulation. So, women using ovulation-detection kits based on monitoring of urinary LH surge may be missing their most fertile days."

What to Do: For more information on how to determine your most fertile time, visit Planned Parenthood. To learn more about pregnancy, check americanbaby.

SOURCES: David Dunson, Ph,D., researcher, National Institute of Environmental Health Sciences, Research Triangle Park, N.C.; Jaime Grifo, M.D., director, Division of Reproductive Endocrinology, New York University Medical Center, professor, obstetrics/gynecology, New York University School of Medicine, New York City, and president, Society for Assisted Reproductive Technology; April 30, 2002, Human Reproduction
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