Early ultrasound technology has revealed that not every multiple gestation results in the birth of twins or triplets. Many pregnancies that begin with two or more embryos ultimately proceed as a singleton pregnancy. This phenomenon is known as the “vanishing twin,” where one fetus, detected early, stops growing and seemingly disappears. The term describes a clinical event where a twin or other multiple is lost in the womb, often without the mother realizing a second embryo existed. It is a form of early pregnancy loss now more frequently diagnosed due to routine first-trimester screening.
Defining the Vanishing Twin Phenomenon
Vanishing Twin Syndrome (VTS) is the spontaneous loss of one fetus in a multiple pregnancy, which results in a singleton birth. This occurs when an embryo, initially confirmed by ultrasound, subsequently fails to develop. The loss almost exclusively happens early in gestation, most often during the first trimester, before the 12th week.
The term “vanishing” refers to the process of reabsorption, where the fetal tissue, gestational sac, and placenta of the non-viable twin are gradually absorbed. This material is typically taken up by the mother’s body, the placenta, or occasionally the surviving twin. The embryo effectively disappears, leaving little or no trace by the time the pregnancy progresses.
The reabsorption is why the mother often remains unaware of the loss, as the body manages the tissue without a complete miscarriage event. In rare instances, if the demise occurs later, the fetal remains might be compressed into a flattened, parchment-like structure known as a fetus papyraceus. The syndrome is fundamentally a reduction from a multiple pregnancy to a single pregnancy.
Statistical Reality: How Often Does It Occur
The actual frequency of the vanishing twin phenomenon is difficult to pinpoint, as many cases go undetected without an early ultrasound. Before the widespread use of ultrasound technology, the only evidence was occasionally finding a remnant in the placenta after delivery. Current estimates suggest that VTS occurs in approximately 20% to 36% of all twin pregnancies initially identified by sonogram.
VTS is more common in pregnancies achieved through Assisted Reproductive Technology (ART). Because multiple embryos are often transferred during ART procedures, and early ultrasounds are standard, the detection rate is greater. In these pregnancies, VTS is estimated to occur in a range of 12% to 30%.
Some researchers theorize that a significant number of all human pregnancies may begin as a multiple gestation, with one twin vanishing before the pregnancy is clinically confirmed. This suggests that the true incidence may be higher than what is currently documented through early ultrasound. However, the data remains limited to pregnancies where a multiple gestation was visually confirmed early on.
Biological Mechanisms Behind the Disappearance
The primary reason for fetal demise in VTS is the presence of chromosomal abnormalities in the embryo. These genetic irregularities prevent the embryo from developing correctly.
Beyond genetic issues, problems with the placenta or umbilical cord can also contribute to the loss. This includes poor implantation into the uterine wall or issues with the shared blood supply in some twin types. The loss may be triggered by a failure to establish sufficient nourishment or a viable connection for continued development.
Once the twin’s heart activity ceases, the body initiates the process of reabsorption. The fetal tissue undergoes necrosis and is broken down into smaller components, which are then absorbed by the maternal bloodstream. This natural mechanism allows the body to clear the non-viable tissue, which is why the vanishing twin often leaves no physical trace by the time of birth.
Effects on the Mother and Surviving Fetus
When VTS occurs in the first trimester, the physical impact on the mother is often minimal. Some women may experience mild symptoms similar to a threatened miscarriage, such as light vaginal spotting, mild cramping, or a temporary plateau in the rise of pregnancy hormones.
The emotional consequences, however, can be significant, especially if the mother was aware of the twin’s existence through an early ultrasound. The parents may experience a complex mix of grief over the loss of one baby and relief that the other is still viable. Counseling may be helpful to navigate these conflicting feelings and the reality of the loss.
For the surviving fetus, the prognosis is generally excellent if the vanishing occurs in the first trimester. However, some studies suggest a slightly increased risk of adverse outcomes, such as lower birth weight or preterm birth. If the loss occurs later, in the second or third trimester, the risks to the survivor are higher, including potential growth restriction or, rarely, neurological issues, due to the release of inflammatory factors or shared placental circulation complications.