Hi Sara,
I know this information is hard to come by. You cannot get 50 pregnant women to go diving and see what happens in order to conduct a study. I teach a PADI Distinctive Specialty on "Women & Diving", so I have extensively researched this topic, no easy task. For the most part the best the medical community can come up with is research on animals and then a best guess. So here is what I have:
Do NOT go diving. Even if you are trying to get pregnant diving should stop. It is possible for you to me 4 to 6 weeks pregnant before you know your are pregnant. During that time the baby starts to develop. Think about how you off gas nitrogen. You exhale it out of your lungs. If you are pregnant and diving the baby absorbs the same amount of nitrogen the mom does. How does the baby get rid of it since he does not exhale? They do not know exactly how it works through the umbilical cord, but studies show it may off gas differently. I have heard of neurological problems such as blindness occurring. Although not a major study and could be coincidence, I know of only three women who dove while pregnant and all three miscarried. Including me. I did not know I was pregnant yet. Might have happened anyway. Who knows?
In short my point is - I a dive worth the risk of a miscarriage or deformed baby?
Until the medical community can say for sure it is not I don't think we should take the chance. Snorkeling is fine and a good alternative.
Take care,
Kristy