It is safe to take Tums. At your first visit they should give a list of meds that are safe to take or to avoid. The heartburn can get pretty intense as you get further along, my OB told me I can take zantac 75mg. In the mean time try to avoid foods that make you gassy and drink lots of water. Your body is changing and your digestive system is slowing down so it is common to get gas pains.
Oh the joys of gas, heartburn, and constipation! Isn't pregnancy fun? You can take tums and I always find that drinking lots of water and eating lots of small meals helps.
Re: gas (?) pains