When to increase your basal

Okay so I need a little advice. I’ve seen on here where starting in the 2nd trimester you start increasing your basal pretty regularly and I think that is where I am almost at. I just increased 2 weeks ago my Levemir from 2 units to 3 units at night and my fastings are 110-125 does it sound like it is time to increase to 4 units? How did you guys know when it was time to increase? Can increasing by just one unit cause severe lows?