The overall somatic effect of twin-to-twin transfusion in utero depends on the type and number of anastomoses and the gestational age at which the transfusional pathways become well established (Baldwin, 1994). The recipient twin is generally, but not always, larger, and its skin surface is duskier, with occasional edema. Histologic assessment of a dozen pairs of parabiotic twins and one set within quadruplets at autopsy, from 23 to 33 weeks of gestation, revealed visceral distinctions useful in understanding the comparative pathophysiology and in the designation as to donor or recipient in the event that the placenta is not available for study. Referent the recipient: (1) the diameter of the umbilical cord, including the fetal end, is greater; (2) the horizontal midventricular plane of the heart shows both chambers to be enlarged in a dilated heart; (3) hypertrophic glomeruli, more so those in the juxtamedullary zone, with maturation of glomerular tufts and Bowman's epithelium; (4) advanced thyroid maturation, with gland enlargement and hypersecretory epithelium; and (5) hypertrophy of pancreatic islets and adrenal cortex. These differences are progressively more marked as gestation proceeds and are evidence of the relative partition of the fetal circulation between monchorionic donor and recipient twins. These findings also suggest a relative hypermetabolic state in the recipient that may serve to reinforce the circulatory misallocation, a factor in the often observed earlier demise of the recipient twin.
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