DS has had problems with this. When he was a few months old he would still have loose stools, but according to xrays, had a lot of stool backed up in the large intestine. Think it's called Meconium Equivilant.
Some people just get a feeling and use Milk of Mag. Others have more difficult problems and are prescribed miralax. Sometimes other measures need to be taken. A friend of mine's granddaughter when diagnosed never had issues with loose frequent stools, but did with constipation.