Complications with Twins

Dr. Elizabeth Pryor answers the question, Why do the ultrasound results keep changing?

ultrasound of twins
Photo: MartinValigursky/


I am pregnant with twins. The first ultrasound showed two sacs but only one heartbeat. A week later, I was bleeding so I went in for another ultrasound. The doctor said to plan on one baby; the other sac never formed. I went back again for another ultrasound one week later and two heartbeats were found and the doctor was able to measure both babies and they measured a week apart. How is this possible, and why do the ultrasounds keep changing?


There are limitations to ultrasounds. I am not sure exactly why they found different things on each ultrasound. There is a thing called "vanishing twin syndrome." This means that the doctor will initially see two sacs and then subsequently only one sac.

It is unusual for two babies to measure a week apart. It is difficult to say what will happen, but I recommend close follow-up by ultrasound.

I wonder if they were unable to see the second baby on the second ultrasound simply because of position. The fact that they saw two heartbeats makes it much less likely that you will lose one of the babies. Generally speaking, once a strong heartbeat has been identified, the chance for miscarriage is less than 2 percent.

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