I don't know if there are any biologists here but I suddenly came up with the question "If a donor organ is rejected by the recipients body's imune system how come the same does'nt happen with an embryo during a pregnancy?"
Asked my mother, who's been in neonatal care for about 30 years. Text message form, so forgive the format.
"That is one of life's miracles - immune system during pregnancy is somewhat suppressed. There is some suspicion that some habitual aborters r actually having rxn to father/fetus HLA - human leukogenic antigen."
I assume, by habitual aborters, she means people who have had multiple miscarriages. That's sometimes referred to as a spontaneous abortion.
EDIT: Sorry, human leukocyte antigen.