Has anyone been told it was a single pregnancy and ended up with twins?

I had an ultrasound at 6 and 9 weeks. First one we didn’t see a whole lot except the heartbeat. Baby was to one side so couldn’t see anything else. Second ultrasound we saw the baby. Both ultrasounds showed one sac and one baby. Now at 20 weeks, another baby AND sac appeared 😳 how did they miss it so long? Never heard another heartbeat at ob appts either.

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