Study Debunks Birth Defect Link to Conception Timing
Belief that aging gametes are to blame is not proven, new research finds
FRIDAY, May 10, 2002 (HealthDayNews) -- Debunking a frightening misconception, scientists have found that babies conceived long after ovulation aren't more likely to suffer Down syndrome or other serious birth defects.
Ovulation, the release of an egg from a woman's ovaries, is a finely scheduled event. And it's true that there's an optimal window of intercourse for conception to take place: a day before or the very day the egg is ejected.
Studies in animals, and inconclusive evidence from people, have suggested that conception with so-called "aging gametes" -- sperm and eggs that have been sitting around in the woman's reproductive apparatus well after ovulation -- might be more prone to genetic birth disorders.
That hypothesis was floated decades ago as a way to explain the sharply higher risk of Down syndrome in babies of women over age 35. The logic went that older women tend to have sex less frequently than younger ones, so their eggs (and their partners' sperm) hang around, get stale and accumulate chromosomal damage.
But the new study shows that eggs and sperm that don't meet until long after ovulation are equally likely to result in normal offspring than those that merge in prime time.
The findings, which appear as a research letter in tomorrow's issue of The Lancet, should comfort couples who practice natural family planning (commonly known as the rhythm method), as well as those who have sex infrequently, the researchers said.
Dr. Joe Leigh Simpson, of Baylor College of Medicine in Houston, led the study, which looked at pregnancy records from more than 1,000 women in the United States, Europe and South America who practice natural family planning.
This method of birth control is popular in heavily Catholic countries where artificial contraception is discouraged by the church. It relies on a mix of timing and telltale body signs, such as changes in body temperature and the consistency of cervical mucus, to alert women to impending ovulation.
Roughly half the women in the study were using the method to avoid getting pregnant. The rest were charting their ovulation cycles to conceive.
More than 100 of the women were excluded from the study, leaving 938. Among the 400 remaining who had optimally timed their conceptions, 11 had babies with major birth defects, or 2.7 percent. On the other hand, 14 infants had such defects among the 538 women with less well-timed conceptions, or 2.5 percent.
Two of the babies in the well-timed timed group had Down syndrome, compared with four among the other women, a statistically insignificant difference.
"We frankly never thought there would be an increase in birth defects other than chromosomal abnormalities, but it was quite logical to think that" such anomalies might occur if the aging gamete hypothesis was correct, Simpon said. "Fortunately for users of natural family planning, we didn't find either."
The researchers acknowledge that their study wasn't perfect. After all, it relied on self-reported ovulation timing, which can be tricky to pin down despite the clues.
Still, Simpson and his colleagues said their analysis was sensitive enough to detect a doubling in the risk of birth defects associated with the timing of conception -- should such an effect have existed.
Stephanie Sherman, a geneticist at Emory University in Atlanta, said the results should help put to rest the aging gamete theory. However, she added, it would have been extremely unlikely for any doctor to caution couples about the potential dangers of delayed fertilization.
Nor does the study help answer the question of why the chances of Down syndrome grow with a woman's age. Some evidence points to errors in proteins that regulate how chromosomes separate during cell division, but so far nothing is concrete, Simpson said.
What To Do
Try the National Down Syndrome Society for more on that condition.
For more on natural family planning, try the Couple to Couple League.
For more on other methods of contraception, check out Planned Parenthood.