Paranoid FTM!!! help!!

I'm 21 weeks and have not had any genetic testing done yet!! Our anatomy scan was perfect, heartbeat and measurements are great and all. my husband and I are young and healthy with no known issues in either family, should I worry too much about things like downs/other birth defects? Would they have known by now if anything is wrong or should I go ahead and have my 2nd trimester screen done? Have any other moms been completely healthy and still had a baby with any sort of issues?