The objective of this study was to determine the chorionic plate vascular anatomy of the monochorionic (MC) placenta in relation to the discordance in fetal growth with or without disparity in amniotic fluid volume. In 58 MC placentae, anastomoses were delineated by dye-contrast injection under optimal physiological conditions. Thirty-two pregnancies were complicated by twin-twin transfusion syndrome (TTTS) (n = 32), of which 16 placentae were from severe disease. Ten pregnancies with fetal growth discordance of >20% and with a normal amniotic fluid index (AFI) were also studied. Sixteen uncomplicated MC pregnancies were used as controls. Severe TTTS placentae (median, m 1; range, r 0 to 2) had significantly fewer anastomoses than those from mild disease (m 2; r 1 to 4; P < 0.01), discordant growth (m 3; r 2 to 6; P < 0.001) and controls (m 5; r 2 to 8; P < 0.001). Placentae from severe TTTS had a single unidirectional deep arteriovenous anastomosis, while milder cases, in addition, had a < or = 1 mm bidirectional superficial arterioarterial (n = 9) or venovenous (n = 6) -type shunts. Multiple arteriovenous anastomoses with a paucity of superficial anastomoses were detected in discordant growth placenta. In contrast, control placentae had multiple shunts which were symmetrical in number, type and size both overall and per placenta. The subchorionic distance in severe TTTS and discordant growth placenta were comparable (m 3.5 cm; r 1.6 to 5.8 cm versus m 3.6 cm; r 2.5 to 5.7 cm), but were greater than the mild disease (m 2.5 cm; r 1.2 to 3.8 cm; P < 0.01) and control groups (m 1 cm; r 0.5 to 2.4 cm; P < 0.001). The perinatal mortality in severe TTTS (57%) was higher than that in the mild TTTS (17%) and growth discordant groups (15%). The paucity of superficial anastomoses with presence of solitary or multiple arteriovenous anastomoses is likely to be associated with severe TTTS and fetal growth discordance of >20% respectively. In contrast, in mild TTTS additional superficial arterioarterial or venovenous channels are present along with single deep arteriovenous anastomoses.