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Just When Can You Become Pregnant?

By Mary Shomon, Based on Information from the NIH

Updated: January 10, 2009

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. Thyroid disease, which frequently causes irregular cycles, can also cause unpredictable fertility.

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.

Some Interesting Facts About Pregnancy and Thyroid Disease:

  • Pregnancy can be detected as early as nine days after conception.
  • A third of all pregnancies end in miscarriage, sometimes before a woman even knows she is pregnant.
  • Undiagnosed, thyroid problems can cause infertility or recurrent miscarriage, making it difficult or impossible for you to get or stay pregnant.
  • 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.
  • Fertility clinics, and ob-gyns often don't even include thyroid testing as part of a fertility testing and rarely include thyroid tests as part of a pre-conception examination
  • Undiagnosed and untreated thyroid problems can worsen your pregnancy symptoms such as fatigue, hair loss, depression, and morning sickness.
  • Undiagnosed, untreated or insufficiently treated thyroid conditions can endanger your pregnancy, increasing the risk of miscarriage, intrauterine growth retardation, pre-term labor, stillbirth, and cognitive problems/mental retardation in your child.
For more tips and information on how to manage your hypothyroidism to have a healthy baby, and a healthy you, visit the Pregnancy & Thyroid Disease Information Center, and read the Thyroid Guide to Fertility, Pregnancy and Breastfeeding Success.

Data Source: NIH

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