Congenital adrenal hyperplasia causes by 21- hydroxylase deficiency (CAH - 210H) is an autosomal recessive genetic disorder caused by the mutation in the CYP21A2 gene. Prenatal diagnosis and treatment will help doctors to perform prenatal treatment for female fetus to prevent masculinization after birth. This study is conducted with the objective to carry out prenatal diagnosis and treatment by dexamethason for pregnant mothers carrying heterozygous mutation in CYP21A2. Nine pregnant mothers carrying mutation in the CYP21A2 gene were treated prenatally with Dexamethasone. The result showed no side effect for pregnant mothers and the newborns birth weight. Prenatal diagnosis showed one fetus is the carries the homozygous mutation 2110C T, six fetuses carry the heterozygous (Q138X, 656A/C G, 1172N, R426C and exon 1 - 3 deletion mutation) in CYP21A2 gene and two fetuses are normal.