Safe to travel to a mexico 4-star resort when pregnant in 2nd trimester? What precautions should I take?

Don't drink water. Be sure not to drink the water or use ice. It is ok to travel in a low risk pregnancy up to 35 weeks gestation.
Safe provided you: Drink only bottled beverages, especially tap or pitcher water. Refuse ice cubes, salads or raw vegetables because of the water + hand hygeine. Same for fruit without peel or ceviche. Most 4-star resorts are safe, however it isn't worth even a minute risk if you are pregnant. Good luck & enjoy the vacation.