Chromosomal abnormalities in the vanished twin.Advanced maternal age (over 35) (This may simply be because multiple pregnancies are more likely in older women.).Usually we don't know what causes vanishing twin syndrome, but factors might include: Most of the time, a twin will vanish without symptoms, but sometimes there will be miscarriage symptoms, such as: Vanishing twins have been increasingly documented since the regular use of ultrasound, but there may be many more cases because the syndrome can happen very early on, even before a first ultrasound. It occurs in about 36 percent of twin pregnancies, about half of pregnancies with three or more embryos, and about 20 to 30 percent of pregnancies achieved with assisted reproductive technology (ART). Vanishing twin syndrome is pretty common. Parasitic twins are very rare, occurring in about one out of every million births. Surgical removal is necessary after birth. The surviving twin may be born with tissue or limbs from the parasitic twin attached to it. Unlike other conjoined twins who both have developed brains and are each functioning, the underdeveloped parasitic twin is completely dependent on the co-twin (called the dominant twin) and cannot survive on its own. As a result, it's not completely formed and is attached to the co-twin. It's a type of conjoined twin that happens when one identical twin stops developing early on. What is a parasitic twin?Ī parasitic twin is unrelated to vanishing twin syndrome. This is called a 'fetus papyraceus.' In very rare cases, the surviving baby takes on some of the lost twin's cells and becomes a chimera – one person with two sets of DNA.

Rarely (if the loss happens after 12 weeks when bones are already formed), the fetus becomes flattened in the uterine membranes and is discovered during delivery. The fetal tissue from the vanishing twin is usually absorbed by the mother and the surviving baby. Usually it occurs early in pregnancy and involves the loss of one twin, while the other twin survives. It's also called disappearing twin syndrome, and it happens when there's a loss of one baby in a multiple pregnancy. Vanishing twin syndrome is a type of miscarriage.

Lost twins at 13 weeks