Can a Hidden Twin Be Detected by Ultrasound Imaging?

Doctor performing ultrasound on pregnant woman

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Ultrasound can tell us a lot about a pregnancy, but it's not always perfect. This is particularly true in the early months of pregnancy. Though it is rare, it is possible to have a "hidden twin" that is not visible during early ultrasound screenings.

Are You Carrying Twins?

Nearly every pregnant person considers the possibility that they could be having more than one baby. Whether they have signs or symptoms that point to twins or just a hunch, the thought is a common one to cross a pregnant person's mind.

The only way to confirm a twin or multiple pregnancy is by visually identifying more than one fetus with an ultrasound. If your health care provider can only see one baby, it is not likely that you're having twins or multiples.

Advances in ultrasound technology have reduced the occurrences of overlooking the second (or third) fetus. A decade ago (or longer), it would have have been easier for a twin to "hide." Today's imaging is far better and allows providers to get a clearer view of the womb.

A second fetus should be clearly visible on ultrasound after 20 weeks. The likelihood that there is another baby hidden in the womb is extremely small at this point.

The Rare Occasion of a Hidden Twin

There are some rare exceptions. Ultrasound provides a picture of the womb, but sometimes the picture can be misleading or misinterpreted.

If there is no evidence of multiples on ultrasound, chances are you are not having twins. However, it is not unheard of for mistakes to be made—particularly when the ultrasound is performed in the first two months of pregnancy.

For example, if your first ultrasound was taken prior to eight weeks of gestation it might have clearly revealed one distinct embryo. Yet, people who have another ultrasound later in the first trimester—or even into the second trimester—have been surprised when the second ultrasound clearly reveals two (or more) developing fetuses.

Another fetus did not simply materialize along the way. Instead, the two-dimensional perspective of the first scan only caught a glimpse of one embryo. The other was shadowed—positioned directly behind the other—and not visible from the view of the ultrasound tool. It is a similar effect to what we see during an eclipse.

A hidden twin happens more often when the fetuses are monochorionic and monoamniotic—sharing both the same placenta and amniotic sac. This forces the two fetuses to be positioned closely together, therefore their shadowed position could not be detected in a quick scan.

What About the Heartbeat?

There are also accounts of health care providers detecting a second heartbeat before they actually see the second fetus on an ultrasound. Not only are these incidents extremely rare, but they do not always mean that there is a hidden twin. It may simply warrant another look via ultrasound.

It's important to remember that an audible fetal heartbeat is dependent on a number of variables. Sometimes fetal heartbeats are not found until near the end of the first trimester.

It is also very difficult to distinguish two heartbeats—especially when the heartbeats are in or near sync, or if your own heartbeat is sending mixed results.

A Word From Verywell

An ultrasound performed later in pregnancy is not likely to overlook a second fetus or a hidden twin. If you remain concerned that you are having undetected multiples, talk to your health care provider.

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  1. UCSF Med Connection. The difference between dichorionic and monochorionic twins.

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