Multiple pregnancies can involve complex developmental scenarios. One such phenomenon, often referred to as a “hidden twin,” occurs when a pregnancy begins with multiple embryos, but only one develops to term. This can be surprising for expectant parents, as the additional embryo’s presence is transient. Understanding this occurrence helps clarify why some singleton pregnancies may have initially started as multiples.
Understanding Vanishing Twin Syndrome
Vanishing Twin Syndrome (VTS) is a condition where one or more fetuses in a multiple pregnancy are lost in the uterus. This typically occurs early in gestation, most often during the first trimester. The fetal tissue is then reabsorbed by the mother’s body, the placenta, or the surviving twin. This reabsorption means only one baby is delivered at birth, despite the pregnancy starting with two or more.
This reabsorption is a natural biological process, reabsorbing non-viable tissue. The term “vanishing” accurately describes how one twin disappears from ultrasound view over time. It is a unique form of early pregnancy loss in multiple gestations.
How Often Does It Occur
Vanishing Twin Syndrome is common, especially with the widespread use of early ultrasound examinations. Estimates suggest it occurs in approximately 20% to 30% of all twin pregnancies. Many cases remain undiagnosed because the loss happens before the first ultrasound, or the initial multiple pregnancy is never detected.
The increased use of assisted reproductive technologies (ART), such as in vitro fertilization (IVF), contributes to a higher incidence of VTS. ART often involves implanting multiple embryos to increase the chances of a successful pregnancy, leading to more initial multiple gestations. Consequently, the rate of one or more of these embryos vanishing also rises. For instance, studies indicate that among pregnancies conceived with ART that initially show two or more gestational sacs, vanishing twin rates can be as high as 36%.
Detecting a Vanishing Twin
Vanishing Twin Syndrome is most commonly detected through early ultrasound examinations. An initial ultrasound might reveal two gestational sacs or two distinct fetuses, but a subsequent scan later in the first trimester shows only one developing fetus. The previously observed second sac or fetus often shrinks or disappears entirely.
In many instances, there are no noticeable symptoms associated with a vanishing twin. However, some individuals might experience mild symptoms such as light vaginal spotting or cramping. These symptoms are often subtle and can easily be mistaken for normal early pregnancy discomforts or implantation bleeding.
Outcomes for Pregnancy and the Surviving Twin
For most pregnancies affected by Vanishing Twin Syndrome, especially when the loss occurs in the first trimester, the surviving twin’s outcome is positive. The remaining fetus typically develops normally, as the reabsorption of the lost twin’s tissue generally poses no risk to its health or development.
After a vanishing twin event, pregnancy usually proceeds as a singleton, with medical monitoring focusing on the surviving fetus’s health and growth. In rare circumstances, if the vanishing occurs later in pregnancy, usually in the second or third trimester, there might be a slightly increased, though still low, risk of minor complications for the surviving twin, such as growth restriction.
Parents who learn about a vanishing twin may experience a range of emotions, including sadness, confusion, or even guilt. Healthcare providers often offer support and counseling to help process these feelings. Understanding that this is a common and natural occurrence can help alleviate emotional burdens.