Recommended newborn formula milk

Hi mummies, Am feeding baby formula as she is a premie/does not have energy to latch. Would like to poll on which brand of formula milk is recommended that will not result in newborn’s hard stools/constipation? Have been on enfamil Post Discharge formula as recommended by our PD but realise stools of baby getting harder... baby has also been fussying quite alot. Any advice is greatly appreciated! Thanks