No medical device is 100 percent accurate all the time, so there’s a chance that an ultrasound missed an extra sac and baby number 3 is playing hide-and-seek. But it’s not very likely. Unfortunately, there is a possibility of something called vanishing twin syndrome, where one of the fetuses is miscarried and reabsorbed into the uterus. No one really knows what causes this and what, if anything, you can do to prevent it. The danger is usually over by around week 12, so by then you can breathe a little easier.
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