All pregnancies come with their own set of concerns and issues that can arise, but it can feel doubly so when it comes to a twin pregnancy.

One thing to be aware of that can happen during the early part of a twin pregnancy, or any multifetal gestation, is what’s known as vanishing twin syndrome.

But, what exactly is vanishing twin syndrome? Why does it occur, and what effect does it have (if any) on the overall pregnancy? We spoke to two OB-GYNs to get a better understanding of the what, why, and how this happens.

Vanishing Twin Syndrome: What is It?  

At six weeks, it can be determined that a woman is pregnant with twins. At the initial ultrasound, a doctor can see if there are two sacs and two heartbeats. However, within the first trimester, one of the twins may, in fact, simply vanish. (In some cases, there can be a twin that disappears even before six weeks, making for a singular pregnancy.)

A vanishing twin is, as Dr. Ellen Delpapa—who specializes in OB-GYN-Maternal and Fetal Medicine at the UMass Memorial Medical Center—explains: “Essentially a miscarriage of one twin. There’s a lot of loss in early pregnancy, and [vanishing twin syndrome] is one of them.”

The twin that does not properly develop, and thus dissolves, will then be absorbed into the other pregnancy, says New York City-based OB-GYN Dr. Jeffrey Yu. The vanishing twin should not have any negative effect on the baby that remains, nor towards the mother.

Additionally, the twin that didn’t take would not be visible at birth, Delpapa explains, as this would only occur if the baby passed after 20 weeks.

Are There Any Signs of Vanishing Twin Syndrome?

“There are no major signs to look out for,” says Delpapa. But if you are experiencing bleeding, spotting, or cramping, you’ll want to be evaluated by your doctor.

While those may be signs of a miscarriage in a singular pregnancy, Delpapa points out that when it comes to twins, the remaining baby is still making hormones, so things like bleeding may not occur during vanishing twin syndrome.

Why Does Vanishing Twin Syndrome Occur?

For some, vanishing twin syndrome could occur due to chromosomal abnormalities or genetics, as can be the case with any miscarriage. And like some miscarriages, there  is sometimes simply no rhyme or reason as to why it happens.

Vanishing twin syndrome is also a rather frequent occurrence, as a 2016 study found that this can happen in 21-30 percent of multifetal gestation pregnancies.

Because of this, both Yu and Delpapa say that when they are initially telling a patient she is having twins, they always mention that there’s always a possibility that one may not develop.

“This sets realistic expectations so there are no surprises and so it’s not as traumatic [if vanishing twin syndrome occurs],” says Yu.

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  • I had this happen to me during my second pregnancy in 1998. The sonographer must have noticed something wrong with the second baby because she refused to acknowledge that there were two heartbeats. I changed OBs, and during a sono a few months later described the incident and was told that this was most likely what happened, but it didn’t have a name at that time.


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