Ans: B. 28 weeks (Ref Williams 24/e p312: COGT 11/e p353; FERNANDO ARIAS 4/e p374).Best time to give anti-D to a pregnant patient is 28 weeks.According to American College of Obstetricians and Gynecologists, 2010:Anti-D immune globulin is given prophylactically to all Rh D-negative, unsensitized women at approximately 28 weeks.Second dose is given after delivery if the infant is Rh D-positive .