does anyone experience a small belly during pregnancy  

Your sister is probably right. I have heard that in most cases, the tummy is usually small and unnoticeable for first time pregnant women, also the fact that you are a slim female maybe adds to this. I don't think you should worry too much, as it is bad for your baby. There's always the doctors and clinics to make sure things are going fine. Good luck on your endeavor.

I think everyday is different in this aspect. When I was pregnant, my belly was very big as my friends said so. But some people are even known they are pregnant because their bellies are not obvious. It is normal. If you are still worried about it, you can go to see a doctor.

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Old folks say that if the mother is still afraid to let everyone know that she is pregnant then chances are the belly will not grow as big. When I got pregnant, I had trouble telling everyone about it. But when everything is settled, everyone has accepted my pregnancy including myself Bam! I went from a flat bellied preggy to a ballooning mom that's about to pop.

I knew of someone who has similar case as yours, her baby was delivered fine and healthy so don't get too stressed out about it. I guess it has something to do with your body shape.

If your worried get checked out hopefully it's nothing but might just be.