I understand your anxiousness to get this over with and to find out once and for all if your baby will be healthy. The reason your doctor is wanting to wait, is to give your baby a little more time to mature and to give baby time to see if anything seen in the first ultrasound has changed. If indicated, then another round of testing will be suggested.
Prenatal testing for Down syndrome falls into two categories: screening and diagnosing. Screening tests, such as ultrasound and maternal blood tests, can help doctors identify women who may hold a greater risk of carrying a baby with a chromosome abnormality such as Down syndrome. Amniocentesis (fluid that surrounds the baby is sampled) and chorionic villus sampling (tissue from outside the sac where the fetus develops is biopsied) are examples of diagnostic tests. Although the results of both tests are highly reliable, they carry a risk of causing a miscarriage.
Screening tests (which is what you have had so far), are offered to all women, not just those with apparent risk factors. Current screening tests for Down syndrome include ultrasound in the second trimester as well as the maternal serum triple screen (blood test). Screening tests tend to have a high false-positive rate because the threshold for declaring a screening test result abnormal is set to capture the largest number of women who truly are carrying an abnormal baby. In other words, most women with an abnormal screening test for Down syndrome go on to deliver a normal infant.
I am listing a site for you to reference, and to give you more information on what is going on with you and your baby. I know it is hard, but please try to relax and don't make any snap decisions until "all" testing is done. Remember..."Most women with an abnormal screening test for Downs syndrome go on to deliver a normal infant."
My best wishes to you and your family, You are in my prayers!