Reproductive immunologists have always followed mainstream immunology and tried to fit the immunology of the maternal-fetal relationship into the paradigms of the day. Attachment of the blastocyst to the uterine lining is thought to trigger the differentiation of the trophectoderm into two layers: an inner cytotrophoblast layer and an outer syncytium of syncytiotrophoblast. Adults homozygous for a highly truncated 'null' allele of human leucocyte antigen G (HLA-G) have been identified. The birth of these individuals suggests that the expression of full-length, membrane-associated HLA-G in trophoblast is not an obligate requirement for pregnancy. The various trophoblast populations differ with respect to both their expression of major histocompatibility complex (MHC) and which maternal immune cells they encounter. In humans, uterine natural killer (uNK) cells are present before implantation and in nonpregnant cycles where the number of uNK cells changes during the course of the menstrual cycle.