ObjectivesWe sought to investigate pulmonary hypoplasia (PH) and associated risk factors in fetuses with congenital conotruncal defect (CTD). MethodsA total of 75 fetuses with CTD (gestational age (GA): 22-32weeks) and 150 normal GA-matched fetuses as the control group were studied. We measured diameters of aorta (Ao); main, left, and right pulmonary artery (PA); and their Z-scores by fetal echocardiography (FE). We also measured the lung area, lung area/chest area ratio (LCR), lung-to-head circumference ratio (LHR), right lung area/head circumference(2) (quantitative lung index, QLI), and Z-scores. ResultsThe PA, left pulmonary artery (LPA), RPA, and their Z-scores and the lung area measurements (except for QLI) were significantly smaller in the CTD group, compared with the normal control group. Subgroup analysis showed the following: (1) CTD with right ventricular outflow tract obstruction (RVOTO) had smaller main and branch PA dimensions and small lung areas in contrast to CTD without RVOTO. (2) CTD with pulmonary atresia had smaller LPA and RPA dimensions but no difference in lung areas. (3) In the common types of CTD, lung area variables were mostly smaller in double outlet right ventricle (DORV) and tetralogy of Fallot (TOF), but there was no difference in transposition of the great arteries (TGA). ConclusionsCTD is associated with hypoplastic main and branch pulmonary arteries as well as PH. Risk factors for PH included defect types (DORV and TOF but not TGA) and RVOTO. It is plausible that pulmonary blood flow, rather than the type of CTD, RVOTO, or pulmonary artery dimensions, plays a key role in PH.