RESUMEN
Background: In man, blood groups are polymorphic genetic systems. Maternal fetal incompatibility phenomena should lead to an elimination rather than a maintenance of these polymorphisms. Apossible mechanism that could explain the persistence of these polymosphisms in natural populations is a selective reproductive advantage of heterozygous individuals. Aim: To explore the relationship between maternal heterozygosity for five blood grups and some obstetrical variables related to gestational success. Material and methods: Using a case control design, to every mother giving birth to a malformed child a consecutive mother, whose offspring was normal, was assigned as control. All women were typified for ABO, Rh, kidd, MNSs and Duffy blood groups. Results: Two hundred two women were studiend. There was only one stillbirth, born from a heterozygous mother for all analyzed loci. Mothers that were heterozygous or homozygous for all loci had a higher frequency of malformed children. Women homozygous for all loci had a higher frequency of living offspring than the rest of the sample. Conclusions: Heterozygous mothers for these genetic systems have a reproductive disadvantage