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Health Capsules
April 2005
Just When Can a Woman Become Pregnant?
Medical textbooks say that an average woman is most fertile from 10 to 17 days after the first day of her menstrual cycle. But NIH researchers have shown what some women have long suspected: many women who think they can only become pregnant during this span become unexpectedly pregnant.

For some women with irregular cycles, there is hardly a day in the menstrual cycle when they are NOT potentially fertile. The window of fertility is most unpredictable for teenagers and women approaching menopause.

Textbooks also usually say that women are fertile for several days after ovulation. Dr. Allen Wilcox of NIH's National Institute of Environmental Health Sciences (NIEHS) has studied fertility extensively. “The basic problem is that ovulation is so unpredictable,” he says. His fertility research has shown that women are fertile only on the day they ovulate and the five days prior to ovulation, not at all after ovulation.

“On average it occurs around day 15,” he says, “but it can happen anywhere from day 8 to day 60 or later.”

Most women don't know when they ovulate, however, and that's where it gets tricky. While women are likely to be fertile between days 10 and 17 of their cycle, 70 percent of women have at least one of their fertile days outside that window, he explains.

Definitions iconDefinitions
Able to get pregnant.

When the egg is released from the ovary.

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Other interesting facts about pregnancy revealed by NIH research:
  • Pregnancy can be detected as early as nine days after conception.
  • Due to natural variations in cycle length, home pregnancy tests may not be able to detect a pregnancy on the first day of a missed period.
  • A third of all pregnancies end in miscarriage, sometimes before a woman even knows she is pregnant.
  Hot Flash News Flash    

Tamoxifen is an effective therapy for some types of breast cancer. However, about 80 percent of women who take the drug get hot flashes. While not life-threatening, hot flashes can be so uncomfortable that people stop taking the medicine. To make this cancer-controlling drug tolerable, doctors can treat tamoxifen-triggered hot flashes with antidepressants like paroxetine. Taking both drugs together, however, may not be a good idea.

Dr. David A. Flockhart, an NIH-funded researcher at Indiana University School of Medicine, knew that the body breaks down tamoxifen and paroxetine with the same enzyme. He wondered whether taking both drugs together might affect blood levels of either or both drugs. To test this, Flockhart and Dr. Vered Stearns of Johns Hopkins University School of Medicine studied 12 breast cancer survivors who had been taking tamoxifen for at least a month and were having severe hot flashes. The researchers gave paroxetine to the study volunteers for four weeks and then took blood samples from them.

Women who took both drugs at the same time had substantially lower levels of a key byproduct of tamoxifen—chemical evidence that paroxetine does affect how the body processes tamoxifen. The effects differed among the women depending on their natural ability to process drugs, which helps explain why tamoxifen's effectiveness can vary among people.

Flockhart cautions that until we learn more, the results of his study should not alter treatment recommendations. The health implications are still uncertain at this point.

Definitions iconDefinitions
Hot Flashes:
Temporary episodes some women get
during menopause in which they feel warm and start to sweat, among other symptoms

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NIEHS Kids' Pages:
Got a budding scientist at home? The Kids' Pages from NIH's National Institute of Environmental Health Sciences (NIEHS) contain more than 1,100 pages of games and music that teach kids about science. There is also a whole series of kid pages in Spanish.
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