The natural cycle embryo transfer consists of transferring the embryos without the previous treatment of endometrial preparation. To do this, the woman's natural cycle is simply controlled and the endogenous release of estrogens and progesterone is taken advantage of.
The problem with this process is that natural hormones are not always sufficient to favor implantation and pregnancy. This will depend on each woman and each case. In surrogacy, natural cycle treatments are not usually done.