What Is Vanishing Twin Syndrome?

Vanishing twin syndrome is the spontaneous loss, or miscarriage, of one developing baby early in a multiple pregnancy, usually resulting in a normal singleton pregnancy. It may also be called disappearing twin syndrome.

Sometimes the term "vanishing twin" is used for any pregnancy in which one baby in a multiple pregnancy is lost while the other survives, even if the twin has not technically vanished. Usually, the term is reserved for a twin that vanishes in the first trimester.

How Common Is Vanishing Twin Syndrome?

Studies suggest that vanishing twin syndrome occurs before the 12th week of pregnancy in around 36% of pregnancies with two fetuses, and in more than 50% of pregnancies with three or more fetuses. Researchers suspect, however, that vanishing twin syndrome may be even more common, because it may frequently go undetected.

In multiple pregnancies that have continued beyond 20 weeks, researchers estimate that about 2.6% of twin gestations and 4.3% of triplet gestations will be affected by fetal death. However, these are generally not considered vanishing twin pregnancies.

Symptoms

In some cases, the loss of the twin may be accompanied by miscarriage symptoms, such as vaginal bleeding and spotting. Other symptoms might include uterine cramps or pelvic pain. These symptoms can occur during the first trimester of pregnancy and may go unnoticed.

If hCG levels are being measured, they may show a slower rise than would be expected in a normally developing twin pregnancy. Low hCG levels in vanishing twin syndrome are usually present from the first days of implantation. It may be reassuring to a parent to know that loss had nothing to do with something they did or did not do.

Diagnosis

Vanishing twin syndrome is typically diagnosed by ultrasound. If an early ultrasound shows a twin pregnancy, but a the parent experiences symptoms of miscarriage or lower-than-expected hCG levels, or only one heartbeat can be detected via Doppler, a healthcare provider may suspect vanishing twin syndrome.

A follow-up ultrasound can determine if one twin has been lost. It may show one normally developing baby alongside a blighted ovum. Additionally, examining the placenta after birth provides useful information as well.

A vanishing twin will likely not be identified if no ultrasounds were done prior to the loss of the twin.

Causes of Vanishing Twin Syndrome

If you lost one of your babies during a multiple pregnancy, there's no reason to believe that it happened because of anything you or anyone else did or did not do. Some cases of vanishing twin syndrome occur because of chromosomal abnormalities in the lost baby, but in other cases, researchers do not fully understand why one twin is lost.

There appears to be a significant increase in vanishing twin syndrome in recent years. Since many vanishing twins would never be detected without early ultrasound, advances in technology could be part of the reason for the increase. In addition, fertility treatments are becoming more common and often increase the chance of multiples. This may also be a factor.

Treatment

Generally, and especially when vanishing twin syndrome occurs in the first trimester, no treatment is necessary for the pregnant parent or the remaining fetus. If a twin dies later in pregnancy, there is a risk of preterm labor, infection, or hemorrhaging. In those cases, healthcare providers will prescribe treatment appropriate for those conditions.

Although there is some debate around this, there's evidence that there may be a greater risk of pregnancy problems after a twin vanishes. Several studies have now looked at babies who had a vanishing twin compared to babies that were conceived without a twin or had a healthy twin.

Several outcomes have been noted, but in general, the risk of birth defects and complications appears to be higher in babies who had a vanishing twin. The increase in risks appears to be most common with the loss of a co-twin between 6 and 8 weeks gestation.

For these reasons, your healthcare provider might want to keep a closer eye on your pregnancy if you had a vanishing twin. However, it's important to note that in most cases of vanishing twin syndrome, the surviving baby is not adversely affected.

If the Twin Doesn't Disappear

Most often the vanishing twin is absorbed by the parent's body so that there is no evidence of the twin at the time of delivery. Sometimes, if a twin vanishes between 15 and 20 weeks gestation, a fetus papyraceous remains. This is a paper-thin remnant of the fetus that may be delivered along with the placenta.

Uncommonly, a teratoma tumor, in which there are some remnants of fetal tissue, such as hair or teeth, may occur in the surviving twin (this is not dangerous to the baby).

Coping

If you have been diagnosed with vanishing twin syndrome, you probably have a lot of mixed emotions. It's normal to grieve the baby you lost while also feeling relief that your other baby survived.

You should not feel that you have to pick and choose between your emotions. You are not dishonoring either baby if you go through a normal grieving process while continuing to eagerly anticipate your healthy baby.

If you are struggling to cope with the loss of the co-twin, speak to your healthcare provider or talk to a mental health professional. They can offer tips on how best to cope with your loss and still embrace your pregnancy.

A Word From Verywell

If you have been told that you experienced vanishing twin syndrome, it can be a challenge to cope with this news—even when the remaining baby is perfectly healthy. But rest assured there is nothing that you did or didn't do that caused the co-twin to vanish. Most of the time, the reason for the loss is unknown.

If you are struggling with anxiety over your pregnancy or the health of the co-twin that you are still carrying, talk to your healthcare provider. They can help ease your concerns about the health of your baby and your pregnancy.