Disappearing twin syndrome or vanishing twin syndrome is a phenomenon of which the causes are mostly unknown. At the beginning of pregnancy, a woman can have two (or even three) embryos in her uterus, but at some point, one of the embryos ceases to develop and only one remains, with practically no trace of the other.
Where does the second twin go?
In the early stages (up to 12 weeks), the tissue is essentially reabsorbed by the mother’s body and the surviving embryo. Less commonly, it is removed along with the placenta after the birth of the surviving baby. Very rarely there are situations when a woman goes to the hospital with all the symptoms of early miscarriage (bleeding, abdominal cramps), and the ultrasound shows that the baby (the only one) is alive and well and is in the uterus in its fetal sac [1].
What causes this to happen?
The vast majority of cases are due to chromosomal abnormalities. In other words, the disappearing twin originally had little chance of survival, and nature chose to focus all the strength of the mother's body on developing one healthy baby [1].
Do twins often disappear?
Until ultrasound was widely used, vanishing syndrome was considered exceptionally rare (first described in 1945). According to recent data, up to 40% of multiple pregnancies turn into single pregnancies by the middle of the second trimester [2].
How will the disappearance of a twin affect the remaining baby?
This question is not yet fully understood. Some studies suggest that a pregnancy with a disappearing twin will go on as a normal single pregnancy, without any additional risks [3]. Other data suggest that the probability of preterm birth in vanishing syndrome remains about the same as it would be in a multiple pregnancy. That is, the remaining twin is likely to be born premature and with low birth weight [2]. Therefore, moms need to be more careful and go to the hospital even when having "training contractions" to try to avoid preterm labor.
