I am a first time mom and 18 weeks 5 days I have been feeling the baby move since 15 weeks 5 days…

I am a first time mom and 18 weeks 5 days I have been feeling the baby move since 15 weeks 5 days the doctors keep telling me the baby looks like a normal size but I'm scared that my baby is going to end up like the rest of my family did I didn't get the gene but they all did it makes me scared. the babies grow really fast once they're born my nephew is 3 months old but the size of an 18 month old