Ive been riding the MRT everyday for the first five months of my pregancy. I think it was only during my fifth month that i used the priority lane. I was showing but not as big and my bump can still be hidden on certain styles pf clothes so there were instances when the guards and other passengers were accusing me. I had to be tougher in telling them my situation. Or, if possible, you can get a pregnancy certificate from your doctor as proof.
The struggle is real in riding the MRT. It really needs some fixing and some more guide rules so the commute would be much easier and less frustrating for everyone. I think you're commute would be fine as long as you don't ride the MRT when it's rush hour.
The train system in Manila needs some serious work, and to make matters worse the people who rise the trains aren't courteous at all, even in priority lanes. I've seen the elderly left standing for an entire train ride.