The York antigen, assigned to the Knops system (KN5-ISBT 022005), is a high frequency antigen present in 90% of the Caucasian and 98% of the African-American population. No cases of anti-Yka in pregnancies have been published. No hemolytic diseases of the fetus and newborn have been observed previously. We report the first case of anti-Yka antibody found in a pregnant woman without fetal anemia, which was monitored by Doppler assessment of peak systolic velocity at the middle cerebral artery. A 36-year-old white woman, gravida 2, para 2 (1994 and 1996) was transfused with two units of packed red cells in 2009. On July 1, 2011 at 13 weeks of gestation of her third pregnancy, “type and screen” showed blood group 0 RhD positive and was found to have one IgG antibody that reacted against all panel red blood cells in the anti-human globulin phase by gel technique. The antibody was identified as anti-Yka and titer was 64. The patient’s phenotype was YK(a–). Peak systolic velocity at the middle cerebral artery, performed by Doppler, at weeks 25, 28, 32 and 34 of gestation did not show fetal anemia. At birth, the newborn was group 0 Rh (D) positive, Yk(a+) with direct Coombs test negative without anemia and hyperbilirubinemia. Our case contributes, as further evidence, to the clinically benignity of the anti-Yka antibody not been a cause of hemolytic disease of the fetus and newborn.