We sought to determine whether direct foramen ovale (FO) assessment or pulmonary venous (PV) flow patterns in fetal hypoplastic left heart syndrome (HLHS) correlate with clinical markers of postnatal left atrial (LA) hypertension severity associated with restrictive FO.Restrictive FO places a newborn with HLHS at high risk of mortality and morbidity.We reviewed the prenatal and postnatal echocardiograms and outcomes of 45 fetuses with variants of HLHS diagnosed since May 1999 to determine whether direct FO assessment or PV flow patterns correlate with clinical LA hypertension after birth.Direct FO assessment in utero showed a poor correlation with postnatal FO size, Pao(2), base excess, and the need for atrial septoplasty (p > 0.05). In 40 fetuses with available PV spectra, three PV flow patterns were observed: 1). continuous forward flow with a small a-wave reversal (velocity time integral [VTI] for reverse/forward flow [VTIR/VTIF ratio or=0.18); and 3). brief to-and-fro flow. Among 19 live-borns, the postnatal FO diameter was smaller in patients with type B than in those with type A flow (1.6 +/- 1.6 mm and 4.5 +/- 2.1 mm, respectively; p = 0.0015), and all patients with type C flow had an intact atrial septum. All three patients with type C flow were critically ill at birth, requiring emergent atrial septoplasty, and two died after heart transplantation, whereas patients with type A or B flow were clinically stable, with only one postoperative death.Prenatal PV flow patterns in HLHS identify the fetus at risk of severe LA hypertension at birth.