Diagnosed with gestational diabetes at 12 weeks. what level of sugar spike is needed to cause birth defects ?

Unknown: In general its not occasional spikes that likely cause these issues. It probably has to do with chronic abnormal sugars. You are almost out of the first trimester, so you are probably ok if it was just a few spikes. However, poor control throughout pregnancy causes lots of issues associated with your delivery, so try to get your sugar control tight so you don't have to worry about that.

Discuss w/ providers: Your OBGYN and other providers need to clearly explain the situation to you and provide you with info on what to do to avoid problems or what to watch for, when to call them or go to an ER. Working closely w/ them is the very best thing you can do. That way you'll have the safest birth and both you and your baby will be glad to be part of a team. Best wishes.