Since tof is congenital, why doesn't fetal ultrasound always find it?

Heart defects: Tetralogy of fallot, a heart defect, can be difficult to diagnose prenatally. The heart is very small in a developing baby, and specific images must be obtained to make that diagnosis. If the baby is in a bad position, or the mother is obese, or the doctor doesn't have a lot of experience diagnosing heart defects in a fetus, tof can be missed.

Expertise required.: The usual views of the fetal heart (so-called 4-chamber heart) can be almost completely normal in tof, thereby rendering the most used screening method for detecting congenital heart defects useless (<1/5 detection rate). Experienced centers will obtain additional view of the great vessels and should be able to detect tof in >2/3 cases.