Parents who used in vitro fertilization (IVF) to conceive their first baby shouldn't assume they'll need to rely on this costly procedure the second time around. A Japanese study of 142 women, published recently in Fertility and Sterility, found that as many as 18 percent were able to conceive their second child naturally.

Possible causes

Doctors aren't sure why the second conception was easier, but they suspect that it had less to do with the IVF treatment than with the pregnancy itself. Some of their theories:

• Fallopian-tube adhesions (strands of scar tissue that can hinder egg entrance into the tubes) tear as the uterus grows, facilitating future conceptions.

• Pregnancy inhibits hormones that contribute to endometriosis, a condition that can lead to infertility.

• The emotional stress of trying to get pregnant may impede women and men's ability to conceive.

To wait, or not to wait

In the study, a woman's age, above all other factors (including the type and duration of infertility), played the most significant role in the couple's ability to conceive naturally.

The bottom line? The researchers suggest moms 33 and younger should generally bank on Mother Nature for two years before returning to IVF, while older moms should resume treatment as early as possible after deciding to have a second child.

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