There are two types of teratology tests: preconception and late pregnancy. Early pregnancy teratology tests refer to early NT screening and are usually performed between 12-14 weeks of gestation. Mid-trimester screening is performed between 20-24 weeks of gestation. In the early stage of pregnancy, NT ultrasound is used to find out the thickness of the fetal transparent layer, which is less than 2.5 cm and more than 2.5 cm indicates the possibility of chromosomal abnormalities. The mid-pregnancy teratology examination is mainly to know whether the fetus has hydrocephalus and heart malformation, and also to know the maturity of placenta and amniotic fluid. If you miss the best time for the screening during pregnancy, you can make up for it with a second screening, which is a very important tool to detect fetal birth defects.