Clarification on Triple marker test report and suggestion please

Patient: Good morning sir/mam,my wife is 18 weeks pregnant.her weight is 66 and she will be 32 years 5 moths old, by the time of negative medical hostory, all are normal from both of our familys.we both are non alcholic, non smokers.recently we have made triple marker test , the test results aretest value ref range for 18 weeksAFP – 51.7ng/mL 34.08-76.61Beta HCG 61426mIU/mL 4720-80100Estriolunconjugated 1.3ng/mL 0.77-2.15Even though the test results are well within in reference intervals, in the report summary they given as TRISOMY 21 high risk and , hcg value is high like that and doctor suggesting for us to do amniocentesis. please clarify me, even though hcg value is in normal range, how the trisomy 21 high risk came. please suggest me, what is my next step to be, safe and healthy.thanks