See the following link for an interesting paper on the ability of the womb to discern between a cloned embryo and an embryo made by means of fertilization. See this link for the article.
Embryonic stem cells from cloned embryos are indistinguishable from those made from embryos made by fertilization. They express the same genes (see DJ Guo et al., Proteomics. 2009 Apr 22, and this article), show the same biological behaviors (see this link for this paper), show normal embryonic stem cell morphology, express key stem-cell markers, and can differentiate into multiple cell types in vitro and in vivo (JA Byrne, Nature 450 (2007): 497-502).
Since embryonic stem cells are made from the internal cells of the embryo (the inner cell mass), the inner cell mass cells from cloned embryos are rather normal (ML Condic, Cell Proliferation 41, suppl 1: 7-19). However, the outer layer of cells (trophectoderm) that engage the endometrium and work with it to implant the embryo into the inner layer of the uterus do not differentiate normally in cloned embryos (DR Arnold et al. Reproduction 132, no. 2 (2006): 279-90). Trophoblast cells in cloned embryos are normal at the early stages (S. Kishigami et al., FEBS Letters 580, no. 7 (2006): 1801-6), but they go on to make abnormal placentas (DR Arnold et al, Placenta 29 Suppl A (2007): S108-10).
Now this paper shows that the differentiating placenta of the cloned embryo does not interact normally with the surrogate mother’s uterus. This is probably one of the main reasons why cloned embryos and fetuses tend to die prior to birth. The endometrial cells of the mothers who were carried the cloned embryos showed substantial variation in the genes they expressed in comparison to endometria that carried in vitro fertilized embryos (S. Bauersachs et al., PNAS 106, no. 14 (2009): 5681-6). Thus cloned embryos fail to properly communicate with the mother’s uterus.
Implantation is a very complex process. It requires cross talk between the embryo and the uterus. Without this cross talk, implantation does not occur successfully. Without successful implantation, the embryo perishes.
Here again we find another reason to not clone humans. We are subjecting them to a process that is less robust than fertilization. The chances of the embryo surviving are far less than an embryo concieved in the usual manner (fertilization). We should simply ban this process in humans overall.