There is very little good information about the risk to the pregnancy of maternal diving. As you can imagine, it would be difficult to do such studies in humans, and the animal data are apparently inconsistent.
Most of the human information is anecdotal, but there is some indication that there may be an increased risk of cardiac and pulmonary abnormalities in fetuses whose mothers dove. There is, as far as I could find, no useful information about whether this is dependent on the depth of the dives or the time in pregnancy when the diving was done.
The UK Sport Diving Medical Committee makes the very reasonable observation that, since diving is purely elective for the vast majority of women, and since there is some weak evidence to suggest an increased risk of fetal malformations, that pregnant women simply avoid diving.
There is no issue of increased pressure across the amniotic fluid. There are no air spaces within the fetus, and the pressure simply equalizes across all fluid-filled spaces. The issues are thought to have to do with bubbling in the fetal circulation, which occurs just as it does in the maternal.