Vanishing Twin Syndrome

As the name suggests, vanishing twin syndrome (VTS) is a condition in which one of the twins or multiple embryos, dies in utero, vanishes, or gets resorbed partially or completely. The result is that a multifetal pregnancy spontaneously becomes a singleton pregnancy, giving the illusion of a vanishing twin. Simply put, the number of embryos conceived as seen during an early pregnancy ultrasonographic scan is different from the number of foetuses delivered. This phenomenon typically occurs in the first trimester in multi-fetus pregnancies.

As vaginal bleeding is a common obstetric complication during the first trimester of pregnancy, VTS may go unnoticed. VTS can also occur as a miscarriage of which the mother is aware, or as vaginal bleeding or spotting in the first trimester, without the mother's knowledge. 

Prevalence of the vanishing twin syndrome

Vanishing twin syndrome is estimated to happen in 36% of twin pregnancies and in half of the pregnancies that begin with at least three or more gestational sacs. It is also estimated to occur in 20-30% of pregnancies initiated through assisted reproductive techniques (ART).¹

Before the use of ultrasound, we had no understanding of the occurrence of VTS. A twin or even more can be lost during a multi-fetus pregnancy before the mother is even aware of the loss or the fact that she was carrying many fetuses. According to Stoeckel's 1945 discovery, the rate of multiple gestations is greater than the rate of their birth, as a twin or even multiple fetuses can be lost during a multi-fetus pregnancy before the mother becomes aware of the loss or the fact that she was carrying multiple fetuses.

What causes vanishing twin syndrome 

In most cases, the cause of VTS occurrence is still unknown. However, certain causes, such as the following, are thought to be linked to the loss of the embryo:

• Maternal age, typically greater than 30 years 
• Abnormal chromosomes in the deceased twin
• Using ARTs, including in vitro fertilisation (IVF)
• Increasing the incidence of multiple gestations.
• Small placentas or other placental structural abnormalities.
• Genetic and teratogenic factors.

Detection and diagnosis

In early pregnancy, accurately diagnosing a missing twin necessitates a thorough ultrasonographic evaluation. This is crucial because any ultrasound artefacts can potentially lead to the misidentification of a second gestational sac or misinterpretation of placental abnormalities, such as placental cysts.

If the twin dies during the first trimester of pregnancy, the chances of the viable co-twin surviving are very good. Additionally, the mother's health remains unaffected, except for some minor vaginal bleeding that may occur as a symptom of the embryo's loss during early pregnancy. The earlier the fetus is lost, the smaller the likelihood is that the mother will notice that the twin has departed, as the odds of the other fetus' survival and the mother's health may not be impacted by the death of the fetus.³

 Before the invention of ultrasound, the diagnosis of a twin, or multiple twin death, was determined by inspecting the placenta after delivery. With the implementation of early ultrasounds, it is now possible to identify twins or multiple fetuses as early as the first trimester. The "disappearance" of a twin may be discovered during a subsequent ultrasound, where for example, only one fetus is seen on the second ultrasound. Additionally, even when an ultrasound shows just one baby in the uterus, some women may experience symptoms that point to a miscarriage. Since early prenatal ultrasonography has been used more widely in clinical settings, VTS has been diagnosed more frequently.⁴

The doctor could check the human chorionic gonadotropin (hCG) hormone levels. hCG is produced by the body during pregnancy. The level of hCG in the body provides information about the status of the pregnancy and whether it is healthy or not. A plateaued hCG level that was previously high enough to support multiple embryos may indicate that VTS has occurred.⁵

Symptoms 

The symptoms that a mother experiences during the first trimester of the pregnancy are comparable to those of VTS. Due to the similarities, it is possible to accidentally overlook that one of the embryos in a pregnancy with twins or triplets has not survived

These signs include:

• Uterus cramps
• Spotting or light bleeding
• Pain in the pelvis
• Backache⁵

Effects of vanishing twin syndrome on the mother and surviving twin

Neither the mother nor the remaining fetus normally exhibits extraordinary clinical signs or symptoms when the loss occurs during the first trimester. Depending on what caused the twin's death, the prognosis for the surviving twin is typically good.

It should be noted that there is a higher risk for the rest of the fetuses to develop cerebral palsy if the twin dies in the second or third trimester.

The fluids contained in the twin's tissues, the amniotic fluid, and the placental tissue may be absorbed by the mother or the surviving embryos when a twin dies after the embryonic gestation period. As a result of increasing pressure from the surviving and developing twin, the deceased twin flattens.

When the dead fetus is delivered, it may be classified as either a "fetus papyraceous" (noticeably flattened due to the loss of fluid and much of the soft tissue) or a "fetus compressus" (sufficiently compressed to be apparent).⁴

Summary 

In the uncommon case of VTS, an embryo out of a set of twins or multiple embryos perishes before delivery. As a result, in cases of twins, the infant inside the womb seems to be a single child, hence the term "vanishing twin". VTS frequently occurs in the first trimester of pregnancy.

It is a surprisingly common condition, as it affects roughly 36% of twin pregnancies and is more common in those involving three or more embryos. It now occurs in pregnancies initiated by techniques such as  IVF at a rate of roughly 20–30%, thanks to advancements in ARTs.

It is not always easy to pinpoint the precise causes of VTS. This phenomenon may be influenced by elements like maternal age, aberrant chromosomes in the deceased twin, fertility therapies like IVF, and early twin loss due to placental problems or genetic elements.

It might be difficult to find the missing twin, especially if the death occurs early in pregnancy. Due to comparable signs and symptoms, including uterine cramps, spotting or light bleeding, pelvic pain, or backache, VTS frequently goes unreported or may be mistaken for a miscarriage.

The mother's and surviving twin's reactions to the loss are different, depending on the period that the twin passes away. Neither the mother nor the surviving twin typically exhibit any notable clinical indications or symptoms if the loss occurs during the first trimester. The surviving twin or twins, however, face a greater amount of danger if the other twin passes away in the second or third trimester, including an increased risk of cerebral palsy.

To sum up, VTS is a rare and occasionally overlooked occurrence in multiple pregnancies that brings to light the intricacies and unpredictabilities of human reproduction. Both medical experts and pregnant parents must comprehend its causes, prevalence, and how it affects the mother and surviving twins.