Maybe you've been trying to get pregnant for months or maybe this was a happy accident -- whatever the case, that little stick says you're on your way to motherhood. To determine how far along you are, your doctor may schedule you for a sonogram -- you find out you're about seven weeks into your pregnancy, and also, congratulations, you're having twins! But when you return for your 12-week checkup, there's no sign of Baby B. What's going on? Vanishing twin syndrome.
While the cause of vanishing twin syndrome is unknown, doctors have some theories: Sometimes the vanishing fetus may have had chromosomal abnormalities; others may be due to implantation problems. In most cases, the vanishing twin does a disappearing act during the first trimester (it's rare, but it can happen later on). When a twin vanishes during the first trimester, the associated tissue and fluids are often reabsorbed by the mother's body, and the remaining fetus is often healthy and carried to term.
The American Pregnancy Association estimates that between 21 to 30 percent of women carrying multiples experience vanishing twin syndrome, whether they're aware of it or not. How could you not be aware? Unless there is a problem, such as a history of miscarriage or other fertility issues, most women don't have their first ultrasound until the second trimester, between 16 and 20 weeks into the pregnancy.