Pregnancy is an immune paradox: it allows maternal (host) acceptance of a semi-allograft (embryo), while it does not cause graft-versus-host or host-versus-graft reactions against the host/mother, or immune suppression. Therefore, the pregnant immunological status is compatible with the desired immune profile in patients undergoing BMT. By replicating the immune profile present in pregnancy in BMT patients, we may be able to reduce the occurrence of GVHD-related morbidity and mortality rates.