BOSTON (AP) — Doctors are reporting an epidemic — of twins. New federal numbers show that nearly half of all babies born with advanced fertility help are multiple births, the vast majority of them twins.

In the five years since the “Octomom” case, big multiple births have gone way down but the twin rate has barely budged. Twins aren’t always twice as nice; they have much higher risks of prematurity and serious health problems.

Now fertility experts are pushing a new goal: One. A growing number of couples are attempting pregnancy with just a single embryo, helped by new ways to pick the ones most likely to succeed. New guidelines urge doctors to stress this approach.

The trend was discussed at a recent American Society for Reproductive Medicine conference in Boston.