Losing one fetus in a multiple gestation while the other pregnancy continues is possible. This event, where a twin pregnancy spontaneously reduces to a singleton pregnancy, is typical when the loss happens early in gestation. It has become more frequently identified since the routine use of early ultrasound technology.
Understanding Vanishing Twin Syndrome
The loss of one twin while the other survives is formally referred to as Vanishing Twin Syndrome (VTS). This occurs when one embryo, initially detected on an ultrasound, fails to develop and stops growing, representing a miscarriage of one member of the multiple gestation. The non-viable fetal tissue is not expelled but is reabsorbed by the mother and the surviving twin’s placenta.
Reabsorption is the most common outcome when the loss happens in the first trimester. The body efficiently breaks down the fetal tissue and the gestational sac, causing the evidence of the lost twin to “vanish” on subsequent scans. The loss is often attributed to chromosomal abnormalities in the deceased twin.
If the loss happens later, typically in the second or third trimester, the body is less able to fully reabsorb the tissue. The deceased fetus becomes flattened and compressed by the growth of the surviving twin and the uterine wall. This flattened, parchment-like remnant is known as Fetus Papyraceus.
Detection and Prevalence
Vanishing Twin Syndrome is common, occurring in 20% to 36% of twin pregnancies detected early in the first trimester. The true incidence is likely higher because many cases occur before a woman knows she is pregnant with multiples. Increased use of early ultrasound, especially in pregnancies resulting from assisted reproductive technologies, has led to more frequent diagnoses.
Diagnosis is primarily made through sequential ultrasound examinations. An early scan might show two gestational sacs or heartbeats, but a follow-up scan will show only one viable fetus. The mother may have minimal or no symptoms, though some experience mild symptoms that mimic a typical miscarriage, such as light spotting or cramping.
Since symptoms are often mild or absent, many women only discover VTS incidentally during a routine scan. For pregnancies without early imaging, the initial multiple gestation might never be known. The remnants of the lost twin may only be detected upon examination of the placenta after the delivery of the surviving baby.
Management and Prognosis for the Remaining Fetus
The prognosis for the surviving fetus depends on the gestational age when the twin loss occurs. When the event happens in the first trimester, the outcome is generally excellent, and the pregnancy proceeds without special management. Tissue reabsorption usually prevents the release of harmful substances into the surviving twin’s circulation.
If the loss occurs in the second or third trimester, the pregnancy is classified as high-risk, requiring closer monitoring. The risk to the surviving twin increases due to potential complications like preterm labor, growth restriction, and neurological damage. If the twins shared a placenta, blood-clotting factors from the deceased fetus can sometimes cross into the surviving twin’s circulation.
Management for a late loss involves increased surveillance, including more frequent ultrasounds to monitor the surviving twin’s growth and well-being. Healthcare providers may also use non-stress tests and biophysical profiles to assess the baby’s health. This monitoring aims to detect signs of distress or complications early, allowing the medical team to intervene if necessary.