Constipation question...

Kb103109

My twin girls are just over 5 weeks old and seem to have a lot of problems with not having a BM everyday.  Their Pedi didn't seem to concerned about it when we saw him the other day.  They are on Similac Sensitive for fussiness and gas (premade, not powder) and I pump and alternate each feeding with breast milk.  My girls seem to poop every 3-5 days.  When it starts to get 4-5 days they get horrible painful gas and start screaming all day.  I give them some gas drops with every feeding and it seems to help some.  I hate hearing them scream in pain like that.  Does anyone have any suggestions to help them be more regular with their bowel movements??