Right now there's three blood tests which are FDA approved to detect chromosome problems in pregnancy. The hormone HCG doubles only early in pregnancy and tends to level off by about 8 to ten weeks of pregnancy. The absolute level they reach by 10 weeks tend to be different. That's a difficult question. A second way of preventing birth defect is for women to take folic acid before they are pregnant.
Folic acid can reduce a group of birth defects called neural tube defects.
These include spina bifida and anencephaly. That's essentially a baby born without a brain. There are certain behaviors which are associated with birth defects. When there's a major birth defect, ultrasound can predict this with great accuracy. But other birth defects are difficult to detect and ultrasound may miss them.