Middle screening, or mid-pregnancy screening, is designed to screen for Down syndrome, trisomy 18, and risk values for neural tube defects. The strategy for mid-pregnancy screening is a combination of serologic markers, including alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG) or free beta-human chorionic gonadotropin (β-hCG), and free estriol (uE₃) triple screening, or the addition of inhibin A to form a quadruple screen, which is used to calculate the risk of morbidity in conjunction with the mother’s age, gestational week, and weight. Screening pregnant women is usually set at 15-20 weeks, with a 60-75% detection rate for Down syndrome and a 5% false positive rate. The method can also be used as a screening method for trisomy 18 and neural tube defects. In the event of an abnormality in the middle screening, further prenatal diagnostic methods such as amniocentesis can be used for further clarification.