and serum testing are the two screening methods for screening Down's Syndrome. In the second trimester quadruple test can be performed but it is not so accurate as the combined test. For diagnostic purpose, Chorionic villus sampling
can be done before 13 weeks of gestation and if pregnancy is beyond 15 weeks as is your case, amniocentesis
can be done, but both these procedures carry a risk of miscarriage
. I suggest you to take advice from your obstetrician for further care. Good luck.