Vanishing Twin Syndrome - Causes, Symptoms, Diagnosis, and Treatment

Introduction

Vanishing twin syndrome occurs in twin pregnancies or pregnancies with multiple embryos. It is a condition in which one of the embryos stops developing. In a multigestational pregnancy, one of the embryos detected during an ultrasound examination partially or completely resorbs. In vanishing twin syndrome, one embryo stops developing, but the surviving embryo continues to develop. This phenomenon can occur in the form of miscarriage, but one of the embryos survives.

What Is Vanishing Twin Syndrome?

Vanishing twin syndrome occurs in twin pregnancies or multifetal (multiple baby) pregnancies. Vanishing twin syndrome refers to the loss of one embryo during the first trimester of pregnancy. In this condition, one of the embryos that was detected during an early ultrasound, disappears and cannot be found in a future ultrasound. The vanishing twin syndrome is hard to diagnose as it can occur before the first ultrasound, in some cases. It is commonly detected in IVF (in vitro fertilization) cases as an early ultrasound is performed during six or seven weeks. About 7 percent to 36 percent of IVF pregnancies involving multiple embryos are diagnosed with vanishing twin syndrome.

What Causes the Vanishing Twin Syndrome?

The exact cause of vanishing twin syndrome remains unknown, but it can be due to the following factors that may be responsible for the loss of the embryo.

• Maternal age is more than 30.

• Chromosomal abnormalities in the vanished twin.

• Invitro fertilization (IVF).

• The presence of multiple gestations.

• Anatomical abnormalities associated with the placenta.

• Genetic and teratogenic (substances that cause congenital disorders in the developing fetus) factors.

What Happens in Vanishing Twin Syndrome?

In vanishing twin syndrome, one of the embryos detected during an early ultrasound stops developing and is partially or completely resorbed. If two or more embryos are detected during an early ultrasound examination, one of the embryos cannot be detected in a future ultrasound scan. One of the embryos stops developing, and the tissues are absorbed by the surviving twin or the mother. The loss of the embryo can occur in the form of miscarriage, in which vaginal bleeding or spotting is observed during the first trimester. It can occur at any stage of development, ranging from the disappearance of an early empty yolk sac to a yolk sac that has developed a fetal pole to a fetus with a heartbeat. As the twin cannot be observed in a future scan, it is referred to as the vanished twin.

The vanishing twin may undergo the following physiological changes:

• Resorption: The exact cause for the resorption of the vanishing twin has not been established. Resorption of one of the twins may occur as early as the seventh week of pregnancy to as late as the twelfth week of pregnancy. However, resorption of the vanishing twin occurs without affecting the other twin. As resorption occurs in the first trimester, the survival chances of the other twin is high.

• Blighted Ovum: A blighted ovum refers to a yolk sac inside which an embryo cannot be visualized during an ultrasound scan. Several studies have shown that most of the pregnancies with the vanishing twin syndrome are found to be anembryonic sac coexisting with a normal gestational sac. The anembryonic sac expels during the first trimester and causes vaginal bleeding. Therefore, it is advised to undergo ultrasound examination before dilation and curettage as this process can cause accidental abortion of the normal twin.

• Fetus Papyraceous: The dead twin becomes mummified and compressed or flattened and is referred to as fetus papyraceous. This state is usually observed in multiple gestations. The dead twin gets flattened in between the membranes and the uterine wall. If the twin vanishes during early pregnancy complete resorption occurs. If resorption occurs after the first trimester, the vanishing twin becomes macerated and may affect the surviving twin. In addition, various fetal malformations in the surviving twin have been found to be associated with the vanishing twin via an unknown mechanism.

What Are the Symptoms Associated With the Vanishing Twin Syndrome?

The signs and symptoms of the vanishing twin syndrome are similar to those of miscarriage symptoms during a normal pregnancy. It involves the following symptoms:

• Abdominal cramps.

• Vaginal bleeding or spotting.

• Pelvic pain.

• Back pain.

How Is a Vanishing Twin Syndrome Diagnosed?

The presence of the vanishing twin syndrome is confirmed during an ultrasound examination. If one of the embryos present during an early ultrasound is not detected on a future ultrasound, a vanishing twin syndrome is diagnosed. Also, the level of the hCG (human chorionic gonadotropin) hormone is assessed to determine the progression of the pregnancy. A drop in the hCG level that was high during the twin pregnancy can be a sign of the vanishing twin syndrome. In addition, the placental examination after the birth of the surviving twin helps provide details about the morphological features of the vanished twin.

How Is a Vanishing Twin Syndrome Treated?

• If a twin is lost in an uncomplicated pregnancy during the first trimester, no treatment is required. Supportive treatment is provided to treat symptoms such as uterine cramps, vaginal bleeding, or pelvic pain. The surviving fetus continues to grow and is delivered normally.

• If complications occur due to the loss of a twin during the second or third trimester, the pregnancy is considered as a high-risk pregnancy and close monitoring becomes essential. In addition, this can result in early labor or may cause various health risks in the surviving twin.

What Are the Complications of a Vanishing Twin Syndrome?

If one of the twins disappears after the first trimester, the existing twin may be associated with the following complications:

• Cerebral palsy.

• Intrauterine growth retardation.

• Premature labor.

• Congenital anomalies.

What to Expect With a Vanishing Twin Syndrome?

If one of the twins vanishes during the first trimester, a typical pregnancy without complications can be expected. However, if the twin vanishes after the first trimester, various complications can occur, and frequent monitoring is required. A vanishing twin syndrome cannot be prevented because the genetic abnormalities that prevent the development of an embryo cannot be cured or prevented.

Conclusion

The vanishing twin syndrome occurs in one in eight multifetal pregnancies. It is common in pregnancies achieved through IVF methods. Many women experience miscarriages during their reproductive period. Therefore, emotional and physical well-being is crucial to focus on the surviving fetus for the next several months. Frequent prenatal visits and close monitoring is essential to prevent the risk of complications in the surviving fetus.