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Figure 6. This diagram shows the difficulty that identical MC twins encounter when the blastocyst (a ball like stage of embryonic development, see 6-A, TC) enters the mother’s womb and imbeds in the nutrient lining (DB). At this stage, each twin is represented by a few cells (the two inner cell masses - ICM). In a single pregnancy (not pictured) the placenta will most likely develop normally (large, with an umbilical cord in the middle of the placenta) if the pole containing the inner cell mass enters the uterine lining first. Any rotation of the blastocyst pole containing the inner cell mass will lead to a smaller defective placenta. Since the cells that develop into the MC twins sit at opposite ends (poles) of the blastocyst, as one twin’s cells imbed perfectly (6 - B), the other twin’s cells move farther away from the lining that provides nutrients from the mother. All MC twin blastocyst implantations, therefore, represent a compromise and are prone to develop placentas that are smaller and unequally shared by the twins. EE = endometrial gland; UV = uterine blood vessel (Diagram courtesy of Dr. Emanual P. Gaziano)