The most common cause of ambiguous genitalia in a newborn is:
More than 90% of congenital adrenal hyperplasia is caused by 21-hydroxylase deficiency. Incidence is 1 in 15000-20000 bihs in most populations. Presents with female pseudohermaphroditism or post natal virilization. Ref: Kliegman, Behrman, Jenson, Stanton, (2007), Chapter 577, "Congenital Adrenal Hyperplasia and Related Disorders", In the book, "Nelson's Textbook of Pediatrics", Volume 2, 18th Edition, New Delhi, Pages 2360-2364
21 hydroxylase deficiency
11b -hydroxylase deficiency
17a -hydroxy alse deficiency
3 b-hydroxysteroid deficiency