Is it safe to take antacids if I'm pregnant?

Yes. Tums (calcium carbonate) is ok to take while pregnant. Quantity will need to be watched out as you can take too much causing illness and possible hospitalization.
Yes. Classic antacids, whether tablets (tums) or liquids (maalox) are certainly safe and sometimes a "life-saver" in the third trimester! However, don't hesitate to mention your symptoms to your ob; worsening heartburn or reflux can be treated or prevented with prescription antacids and ppis (proton pump inhibitors) like Omeprazole (prilosec).