Is mild stomachache serious during pregnancy?

It is difficult to diagnose the cause of your stomachache without personally examining you. It could be a simple case of gastritis (take bland diet, avoiding spicy food, and Tablet Gelusil MPS 2 tablets to be chewed after meals will help). Another common cause is colitis. If you do not have urinary symptoms, it cannot be urinary tract infection. I suggest you report this to your gynaecologist immediately, for a thorough evaluation.