Final answer:
Preimplantation diagnosis is possible during the pre-embryonic stage, which begins with fertilization and continues until implantation in the uterus.
Step-by-step explanation:
Preimplantation diagnosis is possible during the pre-embryonic stage of development, which starts at fertilization and ends when the blastocyst implants in the uterus to become an embryo. This stage involves the initial cell divisions and developmental processes such as fertilization, cleavage, blastulation, and ends with implantation. It is during this stage that preimplantation genetic diagnosis (PGD) occurs, allowing for the analysis of the zygote or blastomere cells for genetic conditions before the conceptus implants in the uterus.