Vanishing Twin Syndrome (VTS) is a phenomenon where a pregnancy that begins with two or more fetuses spontaneously reduces to a single pregnancy. This occurs because one of the embryos or fetuses ceases development and is lost. The increased use of early ultrasound technology has made the diagnosis of VTS more common, revealing a process that often goes unnoticed. Understanding the signs and diagnostic methods is necessary to confirm if a multiple gestation was originally present.
What is Vanishing Twin Syndrome and When Does it Occur
VTS involves the loss of one twin or multiple in a multifetal pregnancy. The non-viable tissue is reabsorbed by the remaining fetus, the placenta, or the pregnant person’s body, making the twin appear to have “vanished.” The majority of VTS cases occur very early in gestation, almost exclusively in the first trimester before the 12th week.
The most common reason for the loss is a chromosomal abnormality in the developing embryo, which prevents it from continuing to grow. The prevalence of VTS is estimated to be between 15% and 35% of all twin pregnancies detected in the first trimester. This condition is particularly common in pregnancies achieved through assisted reproductive technology (ART), such as in vitro fertilization (IVF), occurring in 20% to 30% of these cases.
Physical Signs That May Indicate a Loss
Many people who experience VTS have no physical signs or symptoms at all, meaning the loss is discovered only through routine medical imaging. However, the loss can sometimes trigger symptoms similar to a threatened miscarriage. These signs are often vague and easily mistaken for common early pregnancy symptoms.
Symptoms that may indicate VTS include:
- Light vaginal bleeding or spotting, which is a frequent complication in early pregnancy.
- Mild uterine cramping or pelvic pain as the body adjusts to the loss.
- A temporary drop in the level of human chorionic gonadotropin (hCG) hormone.
- A sudden, unexplained reduction in severe pregnancy symptoms, such as morning sickness.
How Medical Imaging Confirms the Diagnosis
The definitive method for confirming VTS is medical imaging, primarily ultrasound technology. Diagnosis relies on comparison: an initial ultrasound must show two gestational sacs or two fetuses with heartbeats, followed by a subsequent scan showing only one viable fetus. The initial scan is often performed early in the first trimester, especially in pregnancies conceived via ART, which increases the likelihood of detecting the multiple gestation before the loss occurs.
When the loss occurs, a medical professional looks for specific signs on the follow-up ultrasound. These signs include the complete disappearance of one gestational sac, or a collapsed or shrunken sac that previously held a viable fetus. If the loss happens before the first scan, the diagnosis cannot be made, and the pregnancy is considered a singleton from the start. If the loss occurs later in the first trimester, the non-viable fetus may still be visible as a smaller, non-developing structure.
While ultrasound is the most reliable tool, blood tests can offer supporting evidence. A high hCG level consistent with a multiple pregnancy, followed by a plateau or drop, may suggest a loss has occurred. However, the hCG level alone is not definitive, as hormone levels naturally fluctuate, and the remaining fetus continues to produce the hormone. In rare instances, when the loss occurs later, the deceased twin may not be discovered until delivery, where residual fetal tissue is found in the placenta.
Effects on the Surviving Fetus and Future Pregnancies
When VTS occurs in the first trimester, the prognosis for the surviving fetus is excellent, and the pregnancy typically proceeds without complications. The reabsorption of non-viable tissue within the first 12 weeks minimizes risk to the remaining twin. The pregnancy is usually managed as a singleton pregnancy moving forward, requiring no special medical care.
If the loss occurs later in the pregnancy (second or third trimester), the pregnancy is considered higher-risk, and closer monitoring is usually recommended. Late-stage VTS can slightly increase the risk of complications for the surviving twin, including cerebral palsy, restricted growth, or premature birth, though this outcome is rare. The emotional impact of VTS can be complex for parents, involving a mix of relief for the surviving baby and grief for the lost twin. VTS does not typically affect a person’s chances of having a successful subsequent pregnancy.