Foramen ovale size in the normal and abnormal human fetal heart: an indicator of transatrial flow physiology.

We used fetal echocardiography to measure the sizes of the foramen ovale and atrial septum in 80 human fetuses from 17.5 to 38 weeks gestation. Forty-six fetuses had normal cardiac anatomy, 19 had left heart obstructive lesions, and 15 had right heart obstructive lesions. Pulsed and color flow Doppler studies were incorporated when available. We found that normal fetuses had a foramen ovale/atrial septum size ratio of 0.33 +/- 0.04 (mean +/- SD). In all (52%) fetuses in which it was available, pulsed Doppler study revealed bidirectional, but predominantly right-to-left, flow through the foramen ovale. Fetuses with left heart obstructive lesions had a foramen ovale/atrial septum size ratio smaller than normal (0.28 +/- 0.05; p < 0.001). Seven of nine fetuses with left heart obstructive lesions who had color Doppler studies demonstrated reversal of the normal flow pattern, exhibiting unidirectional left-to-right transforamenal flow. Those with right heart obstructive lesions had a larger than normal foramen ovale/atrial septum size ratio (0.47 +/- 0.04; p < 0.001). Nine fetuses with right heart obstructive lesions had color Doppler studies which demonstrated almost exclusive right-to-left transforamenal flow. We conclude that foramen ovale/atrial septum size ratio and Doppler interrogation of transatrial flow are helpful adjuncts in determining the presence of congenital heart disease in utero. In addition, early detection of abnormal foramen ovale/atrial septum size ratio may predict whether left or right ventricular development will be impaired throughout gestation, even before gross disparity of ventricular size is apparent.