New York sidewalks won’t always be packed with double and triple strollers. The multiples boomlet caused by in vitro fertilization—twin rates have increased 70 percent since 1980—may soon be a thing of the past. Since doctors can’t always tell which embryos are the healthiest, the standard procedure is to transfer several at the same time to increase the odds of a pregnancy. But, as Isaac Kligman, a fertility specialist at Cornell, notes, “multiples are a consequence, not a goal.” A multiple pregnancy is high risk for everyone involved, not to mention expensive. Today, some clinics offer parents-to-be the option of pre-implantation genetic screening (PGS), in which a cell is extracted from the tiny embryo, allowing chromosomes to be analyzed for abnormalities. Jamie Grifo, director of NYU’s Fertility Center and the first in the U.S. to have a successful delivery from a PGS embryo, notes that the still-controversial procedure is not without risk: A small percentage of embryos will be destroyed from the testing itself, and, occasionally, one cell’s report does not match up with the rest. But advances in cryo-preservation make it easier to try again. “For certain patients,” says Mark Sauer, director of the Center for Women’s Reproductive Care at Columbia University, “there’s no reason not to put one embryo in, freeze the others, then defrost a second if the first doesn’t work.”