It is medically possible to miscarry one fetus in a multiple gestation pregnancy while the other continues to develop and survive. This phenomenon, where a pregnancy that began with twins spontaneously reduces to a singleton pregnancy, is recognized by medical professionals. The ability to detect this event has increased significantly with the routine use of early, high-resolution ultrasound technology. For many individuals, this occurrence happens without them ever knowing they were initially carrying more than one fetus, and the prognosis for the remaining fetus is often excellent, especially when the event happens early in the pregnancy.
Defining Vanishing Twin Syndrome
The medical term for the loss of one fetus in a multiple gestation is Vanishing Twin Syndrome (VTS). This occurs when one of the embryos or fetuses ceases development and is subsequently reabsorbed by the mother’s body, the placenta, or the surviving twin. This reabsorption process is what gives the syndrome its name, as the tissue effectively “vanishes” from the uterus, often leaving no visible trace on later scans.
VTS is most commonly diagnosed during the first trimester, typically before the twelfth week of gestation. If the loss occurs very early, the mother may experience only mild symptoms like light spotting or cramping, or she may be entirely asymptomatic. It is important to note that VTS is a spontaneous event, unlike selective reduction, which is a deliberate medical procedure performed to reduce the number of fetuses.
The outcome for the surviving twin is usually positive, especially if the twins were dichorionic, meaning they had separate placentas and amniotic sacs. Separate placental systems generally prevent the non-surviving twin’s demise from negatively impacting the blood flow or development of the co-twin. While VTS can occur in monochorionic pregnancies where twins share a placenta, the risks to the survivor are often higher due to potential shared blood vessel connections between the fetuses.
Underlying Causes and Frequency
The primary reason a fetus stops developing in VTS is often a severe developmental issue, most frequently a chromosomal abnormality. These genetic irregularities typically occur at or shortly after conception and prevent the embryo from progressing normally. Other contributing factors can include problems with placental implantation or structural defects in the fetus itself.
The prevalence of VTS is estimated to affect between 15% and 36% of twin pregnancies detected in the first trimester. This wide range exists because many cases likely go undetected if an early ultrasound is not performed. The documented frequency of VTS has risen considerably since the widespread adoption of early ultrasound technology, which allows for the detection of multiple gestational sacs much earlier than was previously possible.
The use of Assisted Reproductive Technologies (ART), such as In Vitro Fertilization (IVF), has also contributed to the increased diagnosis of VTS. Since ART procedures often involve transferring more than one embryo, the rate of initial multiple gestations is higher, leading to a higher incidence of VTS (estimated at 20% to 30% of ART multiple pregnancies). Advanced maternal age, generally defined as over 30 years old, is also associated with an increased likelihood of experiencing VTS.
Implications for the Surviving Fetus and Mother
The implications of VTS for the surviving fetus depend highly on the gestational age at which the loss occurs and whether the twins shared a placenta. When VTS happens in the first trimester, the risk to the surviving fetus is minimal. The deceased tissue is absorbed, and the remaining fetus typically continues to develop without complication, often resulting in a healthy, full-term singleton birth.
If the loss occurs later, during the second or third trimester, the pregnancy is treated as higher risk. The demise of a twin later in gestation can potentially lead to complications for the survivor, including an increased risk of preterm labor, low birth weight, and, in rare cases, neurological injury such as cerebral palsy. This elevated risk is particularly concerning in monochorionic pregnancies, where shared blood vessels can cause sudden blood pressure changes or blood flow disruptions to the surviving twin.
For the mother, physical symptoms are often mild or absent, though some may experience minor vaginal bleeding or cramping, which can be mistaken for a threatened miscarriage. Even without severe physical symptoms, mothers who experience VTS may face significant emotional recovery, as the loss is still a form of miscarriage. Furthermore, studies indicate that pregnancies involving VTS may have a slightly higher risk of adverse maternal outcomes later in the pregnancy, such as gestational diabetes or preterm labor, compared to those that started as singletons.
Detection and Medical Monitoring
Vanishing Twin Syndrome is most frequently detected during a routine prenatal ultrasound. An early ultrasound may reveal two gestational sacs or two heartbeats, clearly indicating a multiple pregnancy. A subsequent ultrasound, typically performed a few weeks later, will then show only one viable fetus, leading to the diagnosis of VTS.
Maternal hormone levels, specifically human chorionic gonadotropin (hCG), may initially be elevated due to the presence of two embryos. Following the loss of one twin, these hormone levels might drop slightly, but they usually remain high enough to support the ongoing pregnancy. However, the definitive diagnosis relies solely on the visual evidence provided by the ultrasound imaging.
Once VTS is confirmed, the standard medical approach involves close monitoring of the surviving twin, typically through serial ultrasounds to track growth and well-being. If the loss occurred early in the first trimester and the pregnancy is dichorionic, no specific treatment is usually necessary, and the medical team focuses on supportive care and reassurance. If the loss occurred later, or if the twins shared a placenta, the pregnancy is monitored more intensively for potential complications.