June 29, 2004

Scientific Secrets of Getting Pregnant

Until epidemiologist Allen Wilcox conducted his recent North Carolina Early Pregnancy Study, science did not know when, on average, women ovulate or even how many days during each cycle women are fertile.

Wilcox, chief of the Epidemiology Branch at National Institute of Environmental Health Sciences in Research Triangle Park, N.C., shared the findings from his study in a June 26 talk at the Johns Hopkins Bloomberg School of Public Health. Its title: “Everything You Need to Know in Order to Get Pregnant Without Hardly Trying.”

His study recruited 221 women who were not pregnant but were trying to get pregnant. The participants were asked to keep a daily diary of when they menstruated and when they had sex. Participants also collected daily urine samples, which were used to measure their hormone levels throughout the study.

The study revealed some surprising answers to the most basic questions about getting pregnant.

How many days, on average, is a woman fertile during her cycle?

By comparing hormone levels, menstrual cycles, when couples had sex, and pregnancies (155 during the study), researchers estimated that a woman is fertile for a total of six days during each cycle—five days before ovulation, plus the day of ovulation itself.

When does ovulation take place within the cycle?

Wilcox, MD, PhD, MPH, and his colleagues found that the earliest ovulation took place on day 6 of a cycle (a cycle begins on the first day of bleeding) and the latest on day 60 (in a woman who had missed a period). They determined:

10 percent of participants were fertile at day 6

One-half were fertile between days 11 and 14

10 percent were fertile at day 22

5 percent were fertile when expecting their next period.

In other words, said Wilcox, “Even women who have very regular cycles will have remarkably unpredictable ovulation days.”

What's the probability of getting pregnant if a couple has sex on a fertile day?

The researchers found that, if a couple has “regular” sex (defined as 2 to 3 times a week, when a couple is in their 20s), 25 to 30 percent conceived on the day before ovulation. “Since the six fertile days are widely distributed across the menstrual cycle,” he explained, “if you pay no attention to when you're fertile, you'll probably still be close to maximizing your chances.”

What’s the probability of having sex on a fertile day?

Interestingly, the frequency of intercourse rose during the six days the women were most likely to be fertile. The researchers also recruited a group of 69 women who were not trying to get pregnant (38 had intrauterine devices [IUDs] and 31 had tubal ligations), and the frequency of intercourse in these women also peaked during their six fertile days.

Summing up his findings, Wilcox said, “What do you need to know in order to get pregnant? Nothing. Biology is on your side: It's worked for so many generations. Unless you have infertility problems, my advice is 'don't worry.'”—Rod Graham