It's the first ultrasound of your pregnancy. As the sonographer glides the wand over your abdomen, she delivers a shocker: You're carrying twins!

You go home and adjust to the news, only to receive a second shock at your next ultrasound a few weeks later: There's only one heartbeat now. The twin embryo has mysteriously and silently disappeared.

The phenomenon – aptly named "vanishing twin syndrome" – is not your normal miscarriage. "A typical miscarriage causes bleeding and loss of tissue," says Robert Wool, associate professor of obstetrics and gynecology at Tufts University in Boston. "But with a vanishing twin, there are generally no signs."

The woman continues to have pregnancy symptoms and missed periods, because after all, she's still pregnant. As for the disappearing embryo, it's absorbed back into the mother's body.

While not exactly common, vanishing twin syndrome is less rare than was once thought. "Women are getting ultrasounds earlier, and the ultrasounds have better resolution," says Wool. "So physicians are catching more of these cases." In other words, doctors are seeing these vanishing twins – the shooting stars of the womb – before they vanish.

Why does it happen? As is so often the case with fetal loss, genetic abnormalities (such as extra chromosomes) are probably to blame in most cases. For this reason, Wool speculates, most vanishing twins are fraternal rather than identical.

In fact, this tendency for an embryo to go AWOL is one reason that some doctors implant several at once during in vitro fertilization. As Wool explains, "If five embryos are put into the uterus, the hope is that one or two survive." Some embryos will fail to implant, but for the others, "doctors are counting on the reality of vanishing twins."

Most of the time the pregnancy continues normally, and the surviving baby bears no evidence of having once had a womb-mate. Every now and then, however, something odd happens: The fetus becomes "mummified" in the uterine membranes and is discovered during delivery. This is called a "fetus papyraceus," and it's exceedingly rare. In other cases, the surviving baby takes on some of the lost twin's cells and becomes a chimera – one person with two sets of DNA.