L.H.
When my son was born, he was tiny (5-lbs 14 ozs and a week overdue). He had bouts of constipation from the beginning. I knew that, with him, he was absorbing most of what he ate and had little waste because he had almost no body fat. I would let him go two days max with no BM and then I would lay him on his tummy with a couple of cloth diapers under his bottom area and gently massage his rectum with a warm washcloth for roughly a minute of so. Worked like a charm. I talked to his doctor about it and he said that if I was going to try to stimulate a BM this would be the best way to do it rather than introducing medications. This way, it stimulates them without medications that they could become somewhat dependant on if used regularly.
Hope this helps.