My doctor explained to me that blood test plus 度頸皮 can check 唐氏 over 75%, so if your family and your husband's family don't have 唐氏 record, you can skip 抽羊水. It is because 抽羊水 has certain dangerous although the technology is very good today. But if you are over 35 years old and you want to 100% sure your baby is healthy, you can 抽羊水.