Likelihood of hidden twins?

Im 17w4d with baby #3. We had one scan at 7ish weeks due to concerns of miscarriage. We only saw one baby at the time but I've heard of women finding out later in pregnancy. I have felt more tired and short of breath than I have with any other pregnancy in the last 3 weeks. I feel like I did at 39 weeks with both of my other kiddos. My family keeps joking it might be twins so now I'm genuinely curious about how common this could be. Or if im just struggling extra hard this time around. Im also plus sized and pregnant and my youngest is about to be 1 so not sure if these could be factors as well.