Hemophilia is X-linked recessive, with a gene spanning more than 186 kb, consisting of 26 exons (9 kb) and 25 introns (177 kb), encoding 2351 amino acids, and more than 46 deletion types (including missense, nonsense and shift mutations) have been identified. The identification of heterozygotes is important for genetic counseling. In the past, heterozygotes were mostly detected by measuring plasma AHG levels or by using the ratio of ⅧR:Ag/AHG. Now, molecular genetic means can be adopted, especially DNA blot hybridization and PCR techniques have been successfully applied, which are equal to prenatal diagnosis and are very effective in preventing the birth of heavy children. The plasma level of FⅧ:C in carriers of hemophilia A (mothers) is generally only 50% of that of normal women (average value 25-75%). The average factor IX level of hemophilia B carriers (mothers) is only 33% of normal, and about 10% of carriers are below 25% of normal, so more hemophilia B carriers have bleeding symptoms. In addition to the lack of factor Ⅸ activity in the plasma of patients with hemophilia B cross-reactive substance-positive (CRH+) type, they also lack antigenicity (cross-reactive substance) and cannot neutralize anti-factor Ⅸ antibodies. There are more variants. They can be tested at the relevant unit. A special fetal microscope is used to obtain pure fetal blood (without mixing with amniotic fluid or maternal blood) in mid-pregnancy to determine whether the fetus is a hemophiliac. Therefore, if the fetal blood FⅧC:Ag value is significantly lower than ⬎1%, hemophilia A can be diagnosed, while a normal value can exclude moderate to severe hemophilia. In recent years, genetic diagnostic methods have been applied, which are less risky and have a higher correct rate, but they are technically demanding and have not yet been promoted.