Taking any medication during pregnancy carries significant risks. Seroquel is a mood stabilizer, which likely is recommended to help maintain an "even" level during pregnancy, but it will not keep you from experiencing the episodes of depression.
Here's the deal, most psychiatric medications are prescribed with a contingency for some form of therapy to assist you in learning how to modulate your mood and recognize the symptoms of a "swing" as they are coming on.
I am a psychologist, not a psychiatrist, so take this with a grain of proverbial salt... If you are not currently involved with a cognitive-behavioral therapist get one! Work with this person over the coming months while on medication to develop some C-B strategies. This will NOT "fix" the problem, but might help you while pregnant. Work with your psychiatrist (I hope this is who is prescribing) and the therapist to determine when you are ready to try again. You will need to wean slowly off the meds b/c the rebound of the med cocktails can be pretty severe.
Sorry to say, but there are no "safe" medications during pregnancy. If you really want another child and cannot go without the stabilizer, then make sure to know the "plan" with your doctors well ahead of time so that you know what to look for and what the warning signs may be. Request that you be titrated to the minimal therapeutic dosage for your body type. Do a quick search b/c there must be some support groups out there for other moms in your situation.