Interspecific pregnancy (literally pregnancy between species, also called interspecies pregnancy or xenopregnancy) is the pregnancy involving an embryo or fetus belonging to another species than the carrier. Strictly, it excludes the situation where the fetus is a hybrid of the carrier and another species, thereby excluding the possibility that the carrier is the biological mother of the offspring. Strictly, interspecific pregnancy is also distinguished from endoparasitism, where parasite offspring grow inside the organism of another species, not necessarily in the womb.

It has no known natural occurrence; however, it can be achieved artificially by transfer of embryos of one species into the womb of another.

Potential applications include carrying human fetuses to term as a potential yet ethically controversial alternative to human surrogate mothers or artificial uteri for gay male couples, mothers with damaged uteri or heterosexual couples that do not want to risk childbirth. It would also provide a sober, drug-free and nonsmoking carrier that is less expensive than human surrogates. For animals, it could be a valuable tool in preservation programs of endangered species, providing a method of ex situ conservation. It could also avail for recreation of extinct species. There have been both successful and unsuccessful examples of interspecific pregnancy in a multitude of different animals: “alpaca and lama (Godke 2001), cow (Bos taurus) and zebu (Bos indicus; Summers et al. 1983), banteng (Bos javanicus) and cow (Bos taurus; Solti et al. 2000), horse, donkey, Przewalski’s horse and Grant’s zebra (Summers et al. 1987; Allen et al. 1993), moflon (Ovis gmelina musimon) and sheep (Ovis aries; Dixon et al. 2007), Spanish ibex (Capra pyrenaica) and domestic goat (Capra hircus; Fernandez-Arias et al. 1999) and Indian desert cat (Felis silvestris) and domestic cat (Felis catus; Pope et al. 1993).”

In the case of these two species of mice embryo transfer was somewhat successful. M. caroli embryos were transferred into M. musculus mice. This produced very low rates of success. It was later discovered that development for the first nine and a half days appeared normal, but then hemorrhaging would occur. This same hemorrhaging was observed in horse-donkey interspecific pregnancy. Researchers believe that this is the body’s immunological response to the incompatibility between the two species. What is interesting though is that though embryos from M. caroli fail to develop in M. musculus females, embryos from M. musculus can survive in M. caroli females. Another important aspect of this research is the use of chimeras (an organism or tissues that contains two sets of DNA). With the use of chimeras, it was found that hemorrhaging was less likely to occur, and "postnatal tissue-specific differential growth" occurred. Research suggests that the death of these interspecific fetuses is most likely tied to maternal immune responses and “failure of local immunoregulation.”

Researchers at the University of Tehran transferred Bactrian camel (an Old-World endangered camelid) embryos into dromedary camels. Dromedary camels were selected for this task due to similarity to the Bactrian camel: 37 pairs of chromosomes, similar placentas, similar gestation periods, similar reproductive physiology, comparable body size, and successful examples of hybridization. Four healthy Bactrian camels were born without any complications out of the ten transferred embryos.

The researchers believe that success depends upon the binding of sperm to an oocyte. Without this successful binding, pregnancy failure is more likely to occur after interspecies embryo transmission. They also concluded that in camels, non-immunological embryo reduction does not occur when more than one embryo is in the uterus. With these findings, the researchers believe that they can save the endangered Bactrian camel and relocate them to areas where other camelids are.

Experiments involving interspecific pregnancy reveal the immunological influence of failed interspecific pregnancies. These immunological influences include: trophoblast (cells formed on the outer layer of a blastocyst (which provides nutrients to an embryo), rejection due to a cell-mediated immunological response, inappropriate interactions between trophoblasts and endometrium (site where blastocysts are implanted or the uterine lining), etc.

Immunologically, an embryo or fetus of an interspecific pregnancy would be equivalent to xenografts rather than allografts, putting a higher demand on gestational immune tolerance in order to avoid an immune reaction toward the fetus. Some mice experiments indicate an imbalance between Th₁ and Th₂ helper cells with a predominance of Th₁ cytokines. However, other mice experiments indicate that an immune response towards xeno-fetuses does not belong to classical cytotoxic T lymphocyte or natural killer cell pathways.