One of the biggest challenges for women trying to get pregnant is determining if and when they should keep trying to conceive on their own, and when it is time to seek medical help. Researchers in the UK have attempted to bring some clarity issue by developing a computer model that predicts women's chances of becoming pregnant in any given menstrual cycle according to their age and how long they've been trying to conceive.
But fertility experts say the formula confirms what we've already known about conception, rather than changing current advice for couples with fertility challenges.
"There is a lot of very good evidence that couples who have been trying longer for a baby without success have less chance of getting pregnant in the next cycle, and that older women have a lower probability of getting pregant than younger women," Peter Sozou, a researcher with the London School of Economics who worked on the formula and its accompanying study, told The Huffington Post.
"Our study makes use of such evidence, and makes what we consider to be reasonable assumptions, which build on a body of previous work," he added.
One focus of the paper, published in the online journal PLoS ONE, is how maternal age affects the chances that a couple having unprotected sex, but not necessarily timing intercourse according to peak ovulation, will get pregnant in the next month.
The study did not find substantial differences in women's chances of conceiving between the ages of 25 and 30, but the odds did drop off for women over 35 years old.
For example, women who are 25 and have been trying to get pregnant for three months have an 18 percent chance of getting pregnant in their next menstrual cycle. By contrast, women who are 40 and have been trying for three months have only a 7 percent chance. After one year of trying, 25-year-old women have a 10 percent chance of getting pregnant in their next cycle; 40-year-old women have a 3 percent shot.
"I think that [the model] validated our current standard," said Dr. Wendy Vitek, director of the fertility preservation program with the University of Rochester's Strong Fertility Center, who did not work on the study. "Couples are generally advised to try and conceive for 12 cycles or one year before they go to a doctor. If you're over 35, and you haven't conceived in 6 cycles, then you should be evaluated."
Vitek said the model does provide strong evidence that women who are over 40 and have been trying to get pregnant might want to think about going to their doctor even sooner, say after just three cycles. In that case, however, patients can't necessarily rely on insurance to help cover treatment. Of the 15 states in the U.S. that provide some coverage for fertility treatments, most require couples try for at least a year on their own before coverage kicks in. Vitek said the model once again raises the question, "Is it unfair to make an individual who is 40 and who has been trying to conceive wait for 6 months before declaring her infertile?"
Dr. James Grifo, director of the New York University Fertility Center, said the new data confirm that younger patients should generally wait a year before seeking fertility treatment, which carries with it the risk of twins or higher order multiples. However, he agreed with Vitek that women in their mid-30s should consider treatment at 6 months, and that women over 40 should consider it even sooner.
But Grifo cautioned that mathematical models are useful for health care providers and "policy wonks," not for individual women trying to better understand their odds of getting pregnant. They do not take into account essential individual factors, such as a woman's ovarian supply and the sperm quality of her partner.
"All we can reasonably hope to do is give probabilities, based on the best science available," said Sozou, the study's author. "It's in the nature of probabilities that we can't say for certain when, or even if, pregnancy will occur in a particular couple."
Fertility experts often estimate that one-third of fertility problems are caused by women's reproductive issues, such as ovulation problems, while one-third can be attributed to male factors, such as low sperm quality or quantity; and one-third are a combination of problems from both partners or are unexplained.
"There's only one test of fertility," Grifo said, "and that's trying to get pregnant. You can look at charts and graphs until the cows come home, but you can't predict the future."