There's an odd occurrence in obstetrics that still has doctors scratching their heads: vanishing twin syndrome. Its exact cause is still unknown to the medical profession, though there are some good theories on why it occurs.

Some women are diagnosed with twins early in their pregnancy only to find that one of the babies has vanished in a subsequent ultrasound. Others are diagnosed with twins, then miscarry one of the fetuses at a very early stage while the other baby goes on to have a perfectly normal delivery. Or, as in Pamela Kock's case, they don't find evidence that another baby was there until the birth of their healthy singleton.

Kock, a mother of two from Cincinnati, Ohio, had an easy and uneventful pregnancy until her 37th week when she developed symptoms of preeclampsia. She was ordered to begin maternity leave and stay in bed. One week later her doctor told her the placenta was no longer working efficiently and labor should be induced.

"After my daughter was born, my doctor pointed out that the placenta was double-lobed, which indicated that there had been, at some point, a twin," says Kock. "She called this phenomenon 'ghost twin' and said it was likely the reason for my complications. This twin must have disappeared very early because there was no indication of a second fetus when I had my first ultrasound at 11 weeks of pregnancy."

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