Down syndrome ?

I was told about a week ago or so my babygirl was measuring small , by the weeks that I had . Wasn’t told why , they said many reasons or no reason . They said maybe Down syndrome or a infection like eating raw meat ( which I don’t), chicken pox , other things . Has this happened to anyone? Mommas with babies with Down syndrome , how where they able to tell your baby had Down syndrome? By sonogram? By blood? They did draw blood from me today and said there was a 98% chance to know from my blood to see if there was Down syndrome in babygirl or not. It doesn’t run in my family nor my husbands , and have never really researched it .i am currently 28 weeks pregnant today . They can also stick a needle through my stomach and grab fluid from the baby’s sack to see if she has Down syndrome but that would further along in the rest of my pregnancy.