How does Vanishing or Missing Twin Syndrome Occur?

Vanishing twin syndrome, or missing twin syndrome, occurs when one of the embryos of a multiple pregnancy disappears in the uterus during pregnancy as a result of a miscarriage and is totally or partially reabsorbed by the mother or the placenta.

This syndrome can occur in the initial phase of pregnancy, when two or more fetuses are observed that grow according to expectations. However, after a few weeks only one of them is still alive and with a heartbeat, while the other has completely disappeared. Embryo and placenta have been reabsorbed without the mother having had any symptoms. Nor does it increase the risk during pregnancy of the live fetus.
It is difficult to have data on the multiple pregnancy rate in which this phenomenon occurs since they are not registered as abortions and therefore there are no official data, but it is estimated between 20 and 30%. In recent years, the rate of multiple pregnancies has increased due to the association with assisted reproductive techniques and the increase in the maternal age when it comes to spontaneous pregnancy.

Causes

In most cases the cause of the loss of one of the fetuses is unknown . In some bichorial pregnancies , with different genetic material in the two fetuses, it has been possible to carry out a genetic study of each of the fetuses. Thanks to this, it has been seen that the lost fetus had chromosomal alterations , which in many cases are incompatible with life and, on the other hand, the fetus that has reached term has normal genetic material.

Consequences according to the trimester of pregnancy

First trimester

When the disappearance of the twin occurs in the first trimester, it usually has no consequences for the surviving fetus. The pregnancy proceeds normally and no more controls are required than those usual in a single fetus pregnancy, nor do they require anything special in the delivery.

Second or third trimester

The consequences can be more serious. The severity of the consequences will depend on the placentas . The twin pregnancies we call monochorionic placenta or if they share dichorionic if the placentas are different .

  • If they share the placenta

Your blood vessels can be in communication. When one of the fetuses in a multiple pregnancy stops beating, there is a sudden drop in blood pressure in that fetus. This can produce a vessel-communicating effect with the still-alive fetus, sending blood across the placenta to the dead fetus.
Depending on the volume of blood sent by the live fetus, it can have a very significant decrease in its blood volume and cause alterations due to lack of oxygen in its tissues. If an organism suffers a sudden decrease in blood volume, its tissues will not receive enough blood and therefore will not receive the necessary oxygen either. If this decrease in oxygen is significant, it can produce ischemic phenomena in fetal tissues and damage them. The most severe consequences can occur if this lack of oxygen occurs in the brain tissues and can lead to cerebral palsy.
For all these reasons, when this occurs, it is important to carry out ultrasound and Doppler controls of the live fetus and to be able to rule out brain injuries or decrease in cerebral blood supply.

  • In bichorial gestations

The complication rate from the death of one of the fetuses in the second or third trimester is much less likely . Fortunately, in most cases where one of the two fetuses in a twin gestation dies before delivery, the surviving fetus is of no consequence.

What is the papyraceous fetus?

When the death of one of the fetuses, instead of being early in the pregnancy, does so later, second or third trimester, this fetus is not absorbed as in the first trimester. The deceased fetus will be compressed towards the walls of the placenta, will lose the amniotic fluid and will remain as dehydrated, it is then called a papyraceous fetus . This fetus is delivered in either isolated delivery or together with the placenta, both in a vaginal delivery and in a cesarean section.
What you should know…

  • It occurs when one of the embryos disappears in the uterus due to a miscarriage and is reabsorbed by the mother or the placenta.
  • This occurs without the mother having any symptoms or increasing the risk during the pregnancy of the live fetus.
  • When the disappearance of the twin occurs in the first trimester, it usually has no consequences for the surviving fetus and the pregnancy develops normally. In the second or third trimester the consequences can be more serious.

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