Approximately two-thirds of twins are dizygotic, or fraternal, and their incidence of 7 to 11 per 1000 births increases with maternal age. They result from simultaneous shedding of two oocytes and fertilization by different spermatozoa. Since the two zygotes have totally different genetic constitutions, the twins have no more resemblance than any other brothers or sisters. They may or may not be of different sex. The zygotes implant individually in the uterus, and usually each develops its own placenta, amnion, and chorionic sac (Fig. 6.17A). Sometimes, however, the two placentas are so close together that they fuse. Similarly, the walls of the chorionic sacs may also come into close apposition and fuse (Fig. 6.17B). Occasionally, each dizygotic twin possesses red blood cells of two different types (erythrocyte mosaicism), indicating that fusion of the two placentas was so intimate that red cells were exchanged.
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