Monoamniotic Twins: One Sac, More Risks

Janet Grant's 7-week ultrasound showed one healthy baby. The same technician at 20 weeks nonchalantly informed her she was having twins.

"I honestly thought she was joking, and this was just some sick sonographer's humor," says Grant, mother of five from Tallahassee, Fla. "My husband exclaimed, 'But you told us 13 weeks ago that there was only ONE!' She was so stunned that she'd missed twins at 7 weeks, she stopped the ultrasound then and there to go look for the old films."

Later that day, the technician tracked them down to tell the Grants that even knowing there was a second baby on the films, she still couldn't find it.

"I asked her if she could tell if they were identical or fraternal," says Grant. "I have identical twin brothers, so I was guessing that mine were, too. She simply said, 'I think they're identical. They look to be in the same sac.' I thought that all identicals were in the same sac. Wrong!"

Monozygotic or identical twinning occurs when one sperm fertilizes one egg. This fertilized egg then divides, resulting in two or more identical zygotes. The timing of the division determines the separateness of the placentas and amniotic sacs. When division occurs after eight days following fertilization, fetuses share a single placenta and a single amniotic sac (monoamniotic). About 5 percent of monozygotic twins are monoamniotic.

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