NT scan measurements

hi parents i had an NT measurement of 3.85mm at 11weeks, now at 18 weeks it is 3.1mm, is it normal? i went for amnio but the results came back as abnormal due to low DNA concentration in the fluid. so dr asked me if i wanna do a 2nd amnio. but based on the fetal biometry, things are looking good.. should i go for the 2nd amnio?