Pregnancy "fetal allograft" ; + "fetal semi allograft" ; + "fetal semiallograft" ; + "fetal semiallogenic" . You might also try dropping the 'semi' if you feel enterprising. These are a small part of the results using GOOGLE 

Immunoresponses during pregnancy and in the perinatal period

Project leader: Professor Rigmor Austgulen

Normal pregnancy: Due to its paternal antigens, the fetus may be regarded as a semiallograft in the maternal organism. Successful reproduction is dependent on both local (intrauterine) and systemic immunosuppressive mechanism from the beginning (implantation) until the end (delivery) of pregnancy. Source

A Review: Suppressed Cell -Mediated Immunity and Monocyte and Natural Killer Cell Activity Follow mg Allogeneic Immunization of Women with Spontaneous Recurrent Abortion

Treatment for women with frequent SRA's is very limited because the cause, or causes, has not yet been pinpointed. Due to the apparent close interactions between the mother and fetus - considered a semiallograft -this system provides an excellent therapeutic target for SRA's. It has been previously shown that pretransplant blood transfusions during tissue grafting enhances graft survival (7). Also, end-stage renal disease patients treated with blood transfusions show a pronounced decrease in Th-1 responses with an increase in Th-2 responses (8). It is hypothesized, then, that immunizing women through blood transfusions could assist in fetal survival if the same responses are induced. Source

Maternal - Fetal Interactions

Histologically, the fetus is an allograft (actually a semiallograft since half of its antigenic makeup comes from the mother). Why is this graft tolerated when a skin graft from the child to the mother in later life would be rejected if the immune system were not suppressed? Source

In natural pregnancies, the embryo/fetus is genetically different from the mother because of carrying a complement of paternal genes. This situation comprises one of the major paradoxes of contemporary immunology, raising the question of how the genetically different embryo/fetus and mother co-exist during gestation. Human pregnancy, where cells from the implanted blastocyst migrate in massive numbers into the decidualized maternal endometrium, presents the greatest challenge.

Over the course of the past two decades, many conditions providing protection to the human fetal semiallograft have been identified. These are provided by both the mother and by the extraembryonic tissues of the embryo/fetus. They include soluble substances such as prostaglandins, progesterone and anti-inflammatory cytokines that include transforming growth factor-p and interleukin-l 0. All of these are produced in the female reproductive tract during pregnancy and are present in the uterus and placenta. Remarkably, the placenta and extraplacental membranes themselves have evolved an array of powerful protective mechanisms. To prevent destruction by maternal complement-fixing antibodies to paternally-derived and fetal antigens, the placenta exhibits high levels of the cell surface regulatory proteins that interrupt the complement cascade. Recent experiments in knockout mice show clearly that in the absence of appropriate complement regulatory proteins, ...... Source (note-PDF file)

Recurrent Miscarriage (Pregnancy Loss)
by Eric Daiter, MD
Alloimmune dysfunction resulting in recurrent pregnancy loss has also been proposed. Allogeneic antigens are molecular structures that occur in different members of the same species and have the ability to elicit an immune response. Normally, a person will reject dissimilar (non-self) tissues or structures from the body using the immune system. In pregnancy, the placenta and growing embryo are not entirely "self" but rather are a result of both the maternal and paternal genetic heritages (referred to as a semi-allograft). The placenta (and pregnancy) has a "privileged" relationship with the pregnant woman that allows for it to escape rejection. The mechanism for this privilege is not known. Source

Immunologic Factors 

One of the most interesting functions of the placenta is the regulation of the maternal immune response such that the fetal semi-allograft is tolerated during pregnancy. Trophoblasts are presumed to be essential to this phenomenon because they lie at the maternal–fetal interface, where they are in direct contact with cells of the maternal immune system. Trophoblasts do not express classic major-histocompatibility-complex (MHC) class II molecules.66 Surprisingly, cytotrophoblasts express more HLA-G,............... Source

mAbs.
The Journal of Immunology,
2000, 164: 61006104.

During pregnancy, the fetus can be considered as a semi- allogenic allograft. However, the fetus is tolerated by the
mother, suggesting an induction of tolerance against paternally derived Ags. The mechanisms that account for such a tolerance are still not clearly understood. The fetus is never in direct contact with maternal tissues; therefore, many of the tolerance mechanisms may occur only at the materno-fetal interface in the placenta. One of these interfaces is the site of implantation where fetally derived extravillous cytotrophoblast cells invade the decidua basalis and encounter maternal immune cells such as NKcells, T cells, and macrophages (1). The lack of expression of HLA-A and -B class Ia molecules on these trophoblast cells and, in contrast, the expression of HLA-G class Ib may contribute to the proper outcome of pregnancy (2). Source - PDF

0review
Since the fetus contains genetic material from both the mother and the father it is said to be semi-allogenic for the mother, this means that the mother and fetus would have different major histocompatability complexes (MHC). This should mean that the fetus would be rejected in a fairly short time as in a transplantation or graft, it would also mean that any future fetus with the same father would be even more quickly rejected by the mother. There evidence to suggest that having different MHC's is beneficial (Roitt, 1991)

As mentioned above the placenta consists of tissues derived from both the mother and the embryo, the fetal part being derived from the embryonic membranes and the maternal part from the uterine wall. (It is this boundary between the maternal and fetal tissues that necessitates the immunologic role of the placenta as the embryo is normally semi- allogeneic to the mother.) This initially small area of fetomaternal apposition is enormously increased during pregnancy as the placenta develops by folding and refolding while it proliferates in parallel with the fetus in order to provide the fetus with the nutrients it needs to develop normally and to remove the waste products the fetus produces. Source

See also Williams Obstetrics 1997, page 20, for a further discussion of the semiallogenic fetal graft. Those who have read my posts in the past have seen my reference to the apple tree as a close analogy of the reproductive process in humans where the blossom eventually leads to the new seed bearer when the apple falls from the tree yet during the full reproductive phase the apple is a part of the tree. It could be sensibly argued that the POC is not a real graft to the body of the woman (or pregnancy unit) because there was never a time when it was not a part of her body even when the trophoblasts suppress HLA, (human leukocyte antigens) to prevent rejection. To claim it is not a part of her body is to deny biological reality.

Just for the purpose of giving some historicity to this paradigm and to support my claim that it is also an ancient concept - and recognizing this has no scientific validity - I would note that the Jewish religion regards the fetus as the equivelant of a limb. During the debate on the so called "Unborn Victims of Violence" act reference was made to the Talmudic interpretation of the relationship between the woman and developing offspring. I quote:

In Judaism, the fetus is viewed as part of the woman’s body and not a separate human being maintaining equal rights. Women are commanded to care for the health and well being of their bodies above all else. 

The Legal Status of the Embryo/Fetus

According to Jewish law, a fetus is not considered a full human being and has no juridical personality of its own. While recognizing the potentiality of becoming human, Rashi, the great 12th century commentator on the Bible and Talmud, states clearly of the fetus "lav nefesh hu--it is not a person." The Talmud contains the expression "ubar yerech imo--the fetus is as the thigh of its mother," i.e., the fetus is deemed to be part and parcel of the pregnant woman's body.