The term “hidden twin” refers to Vanishing Twin Syndrome (VTS), a condition where a multiple gestation pregnancy spontaneously reduces to a singleton pregnancy early in development. Modern, high-resolution early ultrasounds have revealed that VTS is far more common than previously understood, explaining why the rate of multiple conceptions is significantly higher than the rate of multiple births.
What is Vanishing Twin Syndrome?
Vanishing Twin Syndrome is a type of early miscarriage where one embryo or fetus in a multi-fetal pregnancy ceases development and physically disappears. The non-viable fetal tissue is typically reabsorbed by the mother’s body, the placenta, or occasionally by the surviving twin itself, which is why the twin appears to vanish from subsequent scans.
The outcome depends largely on the timing of the event. If the demise occurs very early in the first trimester, the tissue may be completely reabsorbed, leaving no visible trace. When the loss occurs later in the first or second trimester, the fetus may flatten out due to pressure from the surviving twin and the uterine wall, forming a mummified mass known as a fetus papyraceus.
The condition is often asymptomatic for the mother. When symptoms do occur, they are typically mild and may include light vaginal spotting, cramping, or pelvic pain, which can easily be mistaken for common first-trimester discomforts.
How Frequently Does This Occur?
Vanishing Twin Syndrome is a common occurrence in early pregnancy, observed in an estimated 15% to 35% of pregnancies that begin as a twin gestation. The incidence is higher in pregnancies conceived through assisted reproductive techniques, such as in vitro fertilization (IVF), with rates estimated between 20% and 30%.
The diagnosis of VTS has become more frequent with the routine use of early, high-resolution ultrasound technology. Before modern imaging became standard, many cases went completely undetected because the loss often happens before the mother is aware she is carrying multiples. A woman who only receives a later ultrasound may simply be told she is carrying a singleton, never knowing a twin was initially present.
The frequency of VTS is also linked to the number of gestational sacs present initially. In pregnancies that begin with three or more gestational sacs, the likelihood of one or more vanishing can be as high as 50%.
Potential Causes of the Vanishing Twin
The primary reason one twin fails to develop is usually due to inherent developmental issues within the embryo itself. The most significant factor is the presence of chromosomal or genetic abnormalities in the non-viable fetus. These errors often prevent the embryo from developing past a certain stage, leading to its demise.
VTS is typically not caused by the mother’s actions or lifestyle choices. Other contributing factors can include issues with implantation in the uterine wall, problems with the umbilical cord, or inadequate placental function, which restricts the necessary blood supply and nutrients to one twin.
Advanced maternal age (generally considered over 30) is also associated with an increased likelihood of VTS because the risk of chromosomal abnormalities in the egg increases with age. Similarly, fertility treatments increase the incidence of VTS since more than one embryo is often transferred, raising the risk of a multiple gestation that may spontaneously reduce.
Effects on the Remaining Fetus and Pregnancy
For the majority of cases where VTS occurs during the first trimester, the risk to the surviving fetus and the overall pregnancy is minimal. The early loss typically does not affect the health or development of the co-twin, and the pregnancy usually continues without complication as a singleton.
However, if the demise occurs later in the second or third trimester, the risks to the surviving fetus can increase. In these later-stage losses, the surviving twin may face a slightly higher risk of adverse outcomes, such as preterm birth or low birth weight. The late-stage loss can occasionally affect the shared blood supply or placental function, particularly in twins who shared a placenta.
Medical monitoring is adjusted based on when the loss occurs. For first-trimester VTS, no special medical care is usually required. When the loss happens after the first trimester, the pregnancy is often monitored more closely, as a high-risk pregnancy, to ensure the continued health and growth of the remaining fetus.