Vanishing Twin Syndrome: Pathogenesis & Diagnosis!

Vanishing twin syndrome has been recognized for the first time in 1945. The term is used when one of the fetuses disappears in a multiple pregnancy.

What Is Vanishing Twin Syndrome?

The vanishing twin, or fetal resorption, is a fetus from a multiple pregnancy which dies in the uterus in then partially or completely absorbed by the mother or brother. The appearance of this phenomenon is sometimes called vanishing twin syndrome.

Fortunately, if the disappearance of the twin occurs in the first part of pregnancy, his tissues are absorbed by the surviving fetus or the mother`s body, and the pregnancy can evolve normally.

However, if the abortion of one of the fetuses occurs later, the doctors will keep the pregnancy under supervision because of the increased risk to complications, like premature birth, the obstruction of the birth canal or the death of the surviving fetus due to placental rupture or chorioamniotic separation.

Occasionally instead of a complete absorption, the dead fetus may be compressed by the growing brother into a pergament structure, known as fetus papyraceus. If the fetus is absorbed completely, there aren`t any further complications besides the first trimester bleeding.

If the even appears during the second or third trimester, the severe complications may include:

• Premature labor.
• Infection by fetal elimination.
• Bleeding.

Even at the end of pregnancy, a fetus papyraceus can block the cervix and make necessary a C-section.

The vanishing twin can die due to:

• A placenta poorly implanted.
• An abnormality which may determine a major organ failure or their complete lack to the fetus.
• A chromosomal abnormality incompatible with life.

Frequently, the twin is an empty egg which never develops beyond the early stages of embryogenesis.

The frequency of the syndrome is of 1 to 8 pregnancies, and may remain unknown in many cases. The phenomenon appears frequently in pregnancies created as a result of IVF. Ultrasound scans are performed early in these pregnancies, so if the multiple conception took place, it can be noticed more than one amniotic sac in the first few weeks and after a period of time the brothers will disappear.

Why this Syndrome Occurs?

The causes of this syndrome are often unknown. The abnormalities that result from the disappearance of a twin seem to be present from early pregnancy, and not develop along the way.

The analysis of the placenta or fetal tissue often shows chromosomal malformations, while the surviving twin is generally normal. An improper implantation of the umbilical cord might be a cause as well.

Mechanisms of the Viable Twin`s Complications

Cerebral Palsy

Researchers have proposed that vanishing twin syndrome may determine spastic cerebral palsy at the viable brother. A possible mechanism is the transfusion of thromboplastin proteins from the death brother at the viable one, leading to disseminated intravascular coagulation (DIC). Supposedly, DIC results from reversing the blood flow from the death brother to the viable one, carrying the thromboplastin proteins in circulation. This high load with thromboplastin proteins is considered as leading to a status of DIC in viable fetus with damage in the central nervous system.

Another proposed theory is the effect of the high amount of blood lost by the viable fetus against the low vascular resistance system of the death fetus through the placental anastomoses. This transfusion may determine large fluctuations in the intravascular pressures leading to intravascular hemorrhage and cerebral palsy.

Aplasia Cutis

The developing mechanism of aplasia cutis is vascular, with a decreased perfusion in the affected area by the death of the brother.

Signs & Symptoms

Most of the times, this syndrome is asymptomatic, meaning that the expecting mother doesn`t realize that one of her fetuses disappears until she visit the doctor`s office at the regular examination she has to make. However, there are cases where pregnant women might show the signs of a miscarriage and might experience, like:

• Pelvic pain.
• Abdominal cramps.
• Vaginal bleeding.

Decreased hormonal levels in the body due to various blood tests may show the fact that one of the fetuses has been aborted.

First Trimester

Morbidity varies in the appearance of the phenomenon during the first trimester. It`s usually limited. The mother is prone to the development of vaginal bleeding and cramps. If the event occurs late, morbidity can be similar to the one from the second or third trimester.

Second & Third Trimester

Maternal complications include:

• Premature labor.
• Infection through retained fetus.
• Severe puerperal hemorrhage.
• Consumptive coagulopathy and obstruction of birth canal by a fetus papyraceus, which cause dystocia and requires C-section.

Evolution of the Disease

Synchronization of this event affects the prognosis of the viable twin and maternal complications. If the event appears in the second half of pregnancy:

• The fetus may develop cerebral palsy or apalsia cutis.
• The mother may develop premature birth, infections, hemorrhage puerparel, consumptive coagulopathy or birth blocking.

Diagnosis

Before using ultrasounds, the diagnosis of this syndrome was given by examining the placenta after birth. At present, the presence of multiple fetuses is detected during the first trimester of pregnancy. A subsequent ultrasound may reveal the disappearance of one of the babies.

For example, a woman can perform an ultrasound in the 6^th or 7^th week of pregnancy. The doctor identifies 2 fetuses, and tells the woman she`s going to have twins. When she returns to the doctor`s office in a future visit, the doctor only notices the heart beats of a single fetus. Another ultrasound is performed, therefore being noticed the existence of a single fetus.

Sometimes, a woman might experience signs that show the loss of a pregnancy, and the ultrasound shows the presence of one single baby inside the uterus.

This syndrome has been diagnosed lots of times since ultrasounds have started to be used. A frequency estimate shows that this condition occurs in 21% to 30% of the multifetal pregnancies.

Lab Studies

• The recent phenomenon causes the increase of plasma protein A associated with the pregnancy and free beta-HCG.
• Alpha-fetoprotein is increased compared with a normal pregnancy.
• Human chorionic gonadotropin growth rate is lower than in a normal pregnancy.

Imaging Studies

Ultrasound is used to confirm the diagnosis of early twin pregnancy. Monitoring the placenta through ultrasound shows the loss of a fetus in the pregnancy evolution.

Amniocentesis after the diagnosis of a vanishing twin by ultrasound detects XY cells by FISH and PCR.

Chorionic villi sampling is useful if the placenta has a mosaic pattern, but at birth there`s only one result.

Differential Diagnosis

• Miscarriage.
• Deciduous reaction on ultrasound.
• Amniotic cavity observed on ultrasound as a second fetus.
• Chorionic sac considered the second fetus.
• Extraembryonic coelom seen as the second fetus.
• Subchorionic bleeding or hydropic villi change of considered the second fetus.

Can Twins Survive if One Dies?

The death of any of the fetuses in a multiple pregnancy will always be a sad and sometimes unexpected event. But frequently, the surviving fetus`s health isn`t affected and everything will then lead to a healthy delivery. This is specifically true when things happen early in the pregnancy, but it`s also true even when a co-twin`s death occurs after the first trimester of pregnancy.

Among the most essential factors in such cases is whether or not the twins share one single placenta or amniotic sac.

Twins in separate sacs and with separate placentas are considered the most frequent twin types. This happens in around 75% of all twins. Generally speaking, this type of twins has the lowest risks of any complications during pregnancy.

If it may happen for the twins to share the amniotic sac (this occurs in less than 1% of cases), their umbilical cord might become tangled. This may block the flow of blood through one or even both umbilical cords.

Twins which share the same placenta may have connection between their own circulations. In such a situation, the death of one of the fetuses may affect the flow of blood for the other fetus.

At birth, the dead twin`s tissue might be visible, although the more time passes between the death and the birth, the harder it might be to identify. If there`s some time passed, the fetal tissue might become flattened against the placenta of the surviving twin, this being known as fetus papyraceous.

Treatment & Prognosis

If a papyraceus fetus is residual, the pregnancy needs to be monitored carefully by series of ultrasounds for evaluating the viable fetus. The risks are:

• Premature labor.
• Obstruction of birth canal.
• The death of the surviving fetus by chorioamnionitis or placental abruption.

This fetus is at risk of low birth weight and gestational age. Uncomplicated vanishing twin syndrome doesn`t require special medical therapy.

In addition, the loss of a twin brother, the fetus which survives shows an increased risk of cerebral palsy, especially if the syndrome occurs in the second half of pregnancy. Other forms of morbidity reported to the surviving fetus are aplasia cutis or areas of skin necrosis. At brothers connected by placental vascularization, transient hypotension at the viable fetus at the time of death leads to poor perfusion and skin necrosis.