The phenomenon sometimes referred to as a “hidden twin” is medically known as Vanishing Twin Syndrome (VTS). This condition describes the spontaneous loss of one fetus or embryo in a multi-fetal gestation, resulting in a singleton pregnancy. The term “vanishing” comes from the fact that the tissue from the deceased twin is typically reabsorbed by the mother’s body, the placenta, or the surviving twin. While first described in 1945, its prevalence became much clearer with the widespread use of early ultrasound technology.
Prevalence and Timing of Discovery
Vanishing Twin Syndrome is more common than many people assume, particularly in pregnancies where multiples were initially conceived. Estimates of its frequency range widely, but VTS is thought to occur in approximately 15% to 35% of all twin pregnancies detected early. The rate may be as high as 36% in twin pregnancies, and around half of all pregnancies that begin with three or more gestational sacs.
The majority of these events happen very early, typically within the first trimester of pregnancy. Before the routine use of high-resolution ultrasound, most cases went completely unnoticed, as the loss did not result in symptoms like heavy bleeding. The increase in diagnosis is directly attributable to early and frequent ultrasound monitoring, which detects multiple gestational sacs or fetuses before one disappears.
Pregnancies conceived through Assisted Reproductive Technologies (ART), such as in vitro fertilization (IVF), show a particularly high incidence of VTS, estimated between 20% and 30%. This higher frequency occurs partly because multiple embryos are often transferred to increase the chances of a successful pregnancy. Close monitoring of these pregnancies means a greater number of these early losses are visualized and documented.
Biological Mechanisms of Vanishing Twin Syndrome
The underlying reason for the demise of one twin is often rooted in genetic or developmental irregularities. The most common factor identified is the presence of chromosomal abnormalities. These irregularities, which often occur at conception, prevent the embryo from developing properly and lead to the cessation of growth.
Other contributing factors relate to the environment within the uterus and the shared resources between the twins. Placental abnormalities, such as a small placenta or vascular issues, can lead to insufficient nutrient and oxygen supply for one of the fetuses. Improper cord implantation can also compromise the vanishing twin’s ability to thrive.
Once the fetal heart stops, the process of resorption begins, which is the body’s natural way of cleaning up the non-viable tissue. The defunct fetal tissue and gestational sac are gradually absorbed by the mother, the placenta, or the surviving twin. This process causes the twin to appear to “vanish” completely on subsequent ultrasound scans. Since the loss is typically driven by developmental defects in the lost twin, the surviving fetus usually continues to develop normally, especially when the loss occurs in the early first trimester.
Physical and Emotional Outcomes
When VTS occurs in the first trimester, the physical prognosis for the mother and the surviving child is generally excellent. The mother may experience mild symptoms such as slight cramping or minor spotting, but often there are no clinical signs at all. The surviving twin’s development is usually unaffected, and the pregnancy proceeds as a normal singleton gestation.
If the loss happens later in the pregnancy, specifically the second or third trimester, the situation is treated as a higher-risk pregnancy. Later fetal demise can pose risks to the co-twin, including an increased chance of being born small for gestational age or preterm birth. Studies have explored a potential link between later VTS and an increased risk of neurological issues for the survivor, though findings remain mixed.
The emotional impact on the parents can be complex, often characterized by a mix of confusion, relief, and grief. Parents may struggle with the loss of a twin they were only briefly aware of, leading to a feeling of deep, unacknowledged sorrow. Counselors often recommend emotional support and trauma-informed care to help parents process the loss while simultaneously celebrating the ongoing pregnancy.
For the surviving twin, there is debate and anecdotal evidence suggesting potential long-term emotional effects, such as feelings of guilt or an intuitive sense of a missing sibling. While the physical consequences of early VTS are minimal, the psychological landscape for the family warrants compassionate attention and professional support.