The Structure of the Placenta in the Twin Transfusion Syndrome

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Abstract
The structure of the placenta was analyzed in 4 cases of the twin transfusion syndrome. Superficial artery-to-artery and vein-to-vein anastomoses and deep arteriovenous anastomoses in shared cotyledons were studied by injecting the vessels with materials which were both colored and radio-opaque. Superficial anastomoses were found to be much less prominent in the syndrome than in normal monochorial placentas treated in the same way. The significance of this is discussed in the light of Schatz''s concept that superficial anastomoses serve to maintain hemodynamic balance between the 2 circulatory territories. The microscopical structure of the placentas was studied mainly by morphometric methods. These revealed that despite the striking qualitative differences between the territories of donor and recipient twins equally striking quantitative similarities were present. The differences could be largely explained by swelling of the chorion in donor territory, but why such swelling should occur remains obscure. Morphometric analysis also showed that the fetal capillaries in donor territory are only half as wide but several times as long as those in recipient territory. The effects which this may have on blood-flow are discussed in the light of Poiseuille''s law in particular it is argued that the rate of blood flow in donor territory may be sufficiently reduced to be a cause of fetal malnutrition.

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