Modern realtime ultrasound equipment and interpretation have presented physicians with a new type of patient, i.e. the fetus. The fetus suffers all the diseases we know as birth defects, but in their formative stages. Two conditions, namely obstructive uropathy and obstructive hydrocephalus, appear to be amenable in some cases to treatment in utero. Animal models for these conditions have been developed and the first attempts to treat the disorders in humans have been made. While many questions remain unanswered, some initial observations and conclusions can be drawn from this experience.