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When to start trying if you want a specific number of kids

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By which age should couples start trying to have a family, if they have a specific number of kids in mind? A computer model created by researchers in the Netherlands offers answers for couples hoping to have one, two, or three children.

Past research on the effect of age has tended to focus on the chances of a first pregnancy, the researchers noted. The new study, published in the journal Human Reproduction, aims to fill an information gap by specifying the chances of realizing a certain family size.

“Young couples have a general idea that fertility declines with age, but these ideas can be crude and sometimes wrong,” said study coauthor Dik Habbema of the Erasmus University Medical Center in Rotterdam. By replacing a vague idea with more concrete knowledge, he said, the findings might better guide decisions.

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The computer model was built using data on non-contraceptive-using populations spanning 300 years. Having data on couples who tried to have as many children as possible, whether for religious or other reasons, allowed the researchers to assess the odds of having children and to observe how those odds decline with age. The model also incorporated recent in vitro fertilization (IVF) success rates by women’s ages.

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According to the model, women should start trying to conceive by age 32 to have at least a 90 percent chance of having one child. For the same odds of having two children, they should start trying by age 27; for three kids, they should start by age 23. (The model assumes that couples will start trying for the next pregnancy 15 months after the birth of a child.)

For couples willing to use IVF, the ages increase: by 35 for one child, by 31 for two children, and by 28 for three children.

Couples who are willing to accept lower odds of success can afford to wait longer. Trying to conceive by age 34 would give a woman a 75 percent chance of having two children, according to the model, while trying to conceive by age 38 would give her a 50 percent chance. Using IVF would raise these ages by one year. (IVF success rates, like those of unassisted pregnancy, decline with age.)

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While the model doesn’t take men’s ages into account, the results generally hold for couples in which a man is less than 10 years older than a woman, Habbema said. Though women’s ages are more important for likelihood of conception, research has found that children of older fathers may have a higher risk of schizophrenia, autism, and other disorders.

Because the model is based on averages, it doesn’t provide personal predictions. And because IVF success rates are based on data from the Netherlands, women undergoing the procedure in the US may face different odds.

IVF pregnancy rates in Europe tend to be lower than those in the US, perhaps in part because the practice is more heavily regulated in Europe, said Elizabeth Ginsburg, medical director of the Assisted Reproductive Technologies program at Brigham and Women’s Hospital. Success with IVF also depends on the reason for infertility, she adds. While some causes of infertility can be overcome successfully with IVF, others are not so easily bypassed.

Still, Ginsburg said, the study’s take-home message is important.

“It’s good to start early,” she said. “There’s no way to know how easy or hard it will be to get pregnant. Plus, the younger a woman is, the more eggs she has. As women get older, they have relatively fewer eggs capable of completing a pregnancy.”

Ami Albernaz can be reached at ami.albernaz@gmail.com.