Can There Be a Hidden Twin on Ultrasound?

Twin pregnancy when only one baby is seen on ultrasound

Doctor performing ultrasound on pregnant woman
Science Photo Library - IAN HOOTON/Brand X Pictures/Getty Images

Get the answers to frequently asked questions about pregnancy with twins and multiples. Twin Pregnancy FAQ 

Question: Can There Be a Hidden Twin on Ultrasound ?

Answer:

Nearly every pregnant woman considers the possibility that she may be carrying more than one baby. Whether it is due to suspicious signs or symptoms or just a hunch, the thought probably crosses everyone's mind at one time.

The only way to confirm a twin or multiple pregnancy is by visually identifying the multiple fetuses with ultrasound.

If your doctor or caregiver can only see one baby, it is not likely that you're having twins or multiples.

There are some very rare exceptions. Ultrasound provides a picture of the womb, but sometimes the picture can be misleading or misinterpreted, particularly if performed very early in the pregnancy or by an less skilled or incompetent technician. After twenty weeks, a second fetus would be clearly visible on ultrasound, and the likelihood that there is another baby hidden in the womb is extremely minuscule.

Chances are, you are not having twins if there is no evidence of multiples on ultrasound. However, there are some rare exceptions, particularly when the ultrasound is performed in the first two months of pregnancy. For example, my first ultrasound, taken at seven weeks' gestation, clearly revealed one distinct embryo. Yet, my twenty week ultrasound clearly revealed two heads, four arms and four legs.

Did another baby materialize along the way? No, not at all. 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, almost like an eclipse. This is likely because the girls were monochorionic, or contained in a single chorion (sac).

Because they were positioned so closely together, their shadowed position could not be detected in a quick scan.

However, as I mentioned, there was no mistaking the presence of twins at my second ultrasound, at twenty weeks' gestation. Ultrasound performed later in the pregnancy is not likely to overlook a second fetus, or hidden twin. If you remain concerned that you are having undetected multiples, please discuss the issue further with your doctor or medical provider.

More answers to Frequently Asked Questions About Twin Pregnancy

Source:

Morin, L. and Lim, K. "Ultrasound in Twin Pregnancies." JOCG June, 2011. Accessed January 3, 2016. http://sogc.org/wp-content/uploads/2013/01/gui260CPG1106E.pdf

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