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Twins, once a rarity to marvel over, are now a common part of American culture, thanks in large part to increased use of reproductive technology. Twins are conceived naturally just 2 percent of the time; for those who get pregnant with fertility treatments the rate is more than 40 percent.

But as adorable as many of us consider twins, this dramatic rise poses serious health risks, and there are mounting efforts to curb it.

Single-Minded Effort, Expensive Outcome

In a Maryland basement, five sets of 4- and 5-year-old twins clamber among a sea of toys at a party/play date their mothers call a "twinapalooza."

Sharon Bernstein says matching outfits and double-everything do turn heads.

"Women always say, 'Oh, you're so lucky you have twins! I wanted to have twins,' " she says. "And I always go, 'Are ya crazy?!' "

Three of these mothers, including Bernstein, used fertility treatment. What they acknowledge, looking back now, are the incredible risks they took in their drive to become parents — risks that posed a danger to themselves and their children.

"I was...