My son is 5 weeks. I was breastfeeding the first 2 weeks but he went from 8.5 to 7.5 and had to go in for weight checks for two weeks and still he wasn't gaining enough despite him feeding 2 hours every 1.5-2 hours a time and him still being hungry (30 minutes a side twice) started supplementing to put on weight and then just fully did formula. He was doing 2-3oz every 2 hours even over night for 2- weeks and then then Tuesday he ate 4oz every 2 hours then last from 9:30 -now 5oz every 2 hours.