THURSDAY, Feb. 23, 2006 (HealthDay News) -- Thanks to hormone fluctuations, the ovaries of older women tend to release more than one egg at a time, explaining why women are more likely to have fraternal twins as they age.
"We have shown that older women have an increased tendency to multiple, simultaneous ovulations, and as FSH [follicle-stimulating hormone] levels are constantly rising in this age group," said study co-author Dr. Roy Homburg, a professor of reproductive medicine at Vrije University Medical Centre in Amsterdam, the Netherlands.
"We believe that an overshoot of the FSH rise causes an increased frequency of multiple ovulations," he said.
According to other experts, the findings are, in their essence, not new. It's the specificity of the discovery that appears to be novel.
"This has been known -- that older women were more likely to have multiples," said Dr. Jaime Grifo, director of reproductive endocrinology at New York University Medical Center in New York City. "What's unique is the way they catalogue the data. No one has quite catalogued the FSH values the way they did."
The report is published in the Feb. 23 online issue of Human Reproduction.
"Ovarian function declines as women get older, affecting fertility and eventually resulting in infertility," Grifo said. "But in between fertility and infertility is this gray zone, where things happen that are out of character."
That gray area is characterized by a rising incidence of "dizygotic" or fraternal twins. One study found that twinning rates increased by 300 percent between the ages of 15 and 37, with the increase particularly pronounced between the ages of 35 and 39.
Homburg and his colleagues analyzed oocyte (egg) follicle development in 959 cycles involving 507 women undergoing artificial insemination because of unexplained infertility or mild male infertility.
Cycles involving more than one follicle over 14 millimeters in size and, by implication, multiple rather than single ovulations, occurred in 105 women.
Of these 105 women, five were under the age of 30, 45 were aged 30 to 35 and 55 were over the age of 35.
The intervening factor appeared to be FSH, which increased with age in both women with single and multiple egg development but was higher in women who released more than one egg at a time.
"FSH is what the brain stimulates the ovary with to make eggs," Grifo explained. "As the ovary gets older and less responsive, it starts to fail and the brain makes up for that by hitting it a little harder. This causes more than one follicle to be recruited in a setting where generally only one is recruited."
"As women age, the eggs are not as good quality so the FSH increases," added Dr. George Attia, director of the fertility unit at the University of Miami School of Medicine. "But the ovaries can still respond. It's not that they won't respond at all."
While additional studies are always a good idea, Homburg said the researchers are "very confident that we've got it right."
Homburg also said he hoped the paper would make women more aware of fertility's finite nature.
"The trend of today's modern living in the Western world is to delay pregnancy until a later age," he said. "From the age of 35 years, ovaries start off on the long route to menopause, having a decreasing number of eggs and, more importantly, eggs with reduced quality, i.e. incapable of being fertilized by sperm. This leads to increasing difficulties to conceive in the older age group."
Unfortunately, he said, "an amazing number of women do not seem to be aware of these facts and 'miss the boat.'"
And having twins later in life isn't all it's made out to be, Homburg added.
"This also might be a wake-up call, as a twin pregnancy at 40-plus is not always the pleasure it would appear to be at first glance," he said.
More information
For more information on fertility and infertility, visit the American Society of Reproductive Medicine.
