As soon as my little girl was born I started to breastfeed. She latched on and it was great. She had a little jaundice, her pediatrician wanted to see her after a few days of being born. She noticed my Lily wasn't gaining as much weight. I had to do a formula supplement which is fine, I wasn't producing enough breast milk. After a while I started to notice a little blood in her poop. I had switched her formula from Similac to Enfamil. I thought the Enfamil was tough on her tummy. So I went back to the Similac sensitive. Still saw a little bit of blood in her poop. I stuck to breastfeeding only nothing changed so I cut out breastfeedig and did formula only a little difference but there was still blood. I took her back to the Dr and come to find out my Lily is allergic to milk protein. The Dr gave me Similac Alimentum. It's hypoallergenic. it's expensive for a small can but totally worth it. I saw a difference within the first poop. I know probably TMI but if your baby is colicky, non-stop spit up, or acid reflux I recommend you ask your doctor for a hypoallergenic formula. Enfamil has a hypoallergenic formula but my Lily didn't take to it. Both of the hypoallergenic formula smell horrendous. It makes their poop smells rancid, burps are gross but to see her sleep and not be fussy is worth it.