hi welcome to our site read your query that is a very significant question and i appreciate your problem i will try my best to answer your query yes magnesium is known to cause diarrhea it is one of the most common medicine used especially for acidity (heard of digene?) this side effect is not very common but is known as are other side effects like nausea and vomiting however with abdominal pain and loose stools i don t think we can just leave it at side effects i think the possibility of gastroenteritis should be ruled out and you may need antibiotics since you are pregnant i would suggest you meet your obstetrician because medicines can hurt the baby a complete haemogram and stool routine would be useful tests i hope this helps you inform the reports mentioned above (if any) so i can be of help further best of luck (please do understand that some problem remains as online texting cuts down history and examination thereby reducing the effect and efficiency and increases reliance on tests) dr.saumyamittal@gmail.com