My daughter breastfeed fine for nearly two weeks, though often and cluster feeds. I didn't know this was normal and had some new mum anxiety this was so often, as her birth weight was still falling at this point.
She then had an infection on her fingers and toes, which on reflection was probably from the hospital (it was extremely dirty, lots of women delivering and very few midwives) and at 2 weeks old had to have penicillin. This was almost death to the breastfeeding. She immediately had stomach upset, diarrhoea, constant screaming and a lack of interest in feeding then, unsurprisingly. Her birth weight then dipped to just under 10% and she was classed as 'failing to thrive'. The Dr was very nice, very concerned and said we had to supplement with formula immediately or she would be taken into hospital. I could see that this was necessary, her fontanelle was sunken and she was starting a grey/blue-ish tint round her mouth. Of course we were petrified, and very quickly she was on 6 bottles a day, with me feeding her first then 'topping up'. I never thought to ask the Dr for how long and how much to top up, and I doubt if he would have been able to tell me, there seems to be a lack of good knowledge of breastfeeding by medics generally, I found.
We were referred for an appointment with a paediatrician a fortnight later. By then she'd put some weight on with the formula and they though she was doing well. I said how unhappy I was with the formula and was still trying to breastfeed, but by then my daughter was gulping enthusiastically at the bottle and impatient at the breast as it wasn't as fast and my supply was reducing. The hospital said, "Wait here, we'll get our feeding specialist to come and talk to you", who arrived, brandishing charts on how much formula to give at each week! They weren't able to offer any breastfeeding advice other than, 'just put her on the breast then'! While I was sitting nearly crying through this, a passing lady Dr took pity on me, saw I'd given my daughter a bottle while waiting, and then put her on the breast as she was fussing/ crying constantly, and decided she needed infant Gaviscon, as she'd been colicky and refluxy from the start. This did help to some degree after this, though it's harder to give Gaviscon in water from a spoon, while breastfeeding than to give in a bottle of formula.
The upshot of this was, at 6 weeks old, after phoning the NCT for advice and expressing to up my supply, I gradually over the next fortnight, dropped one bottle a day, until she was entirely breastfed during the day and had 2 bottles then one, during the night. I have allergies, so really wanted to continue feeding for as long as possible. This was incredibly hard, I remember at the 6 weeks mark just sitting crying, saying 'I can't do this!' it would have been very useful to know about a growth spurt round then!
I actually managed to feed my daughter for 11 months, until she dropped It herself, a feed at a time. In the early months, she never put much weight on, usually no more than 3-4 oz a week, but was very active and a picture of health.
I should add that my Health Visitor was fantastic and was always a tower of support through everything, helping me sort out which bottle to stop when etc. I don't think I could have managed to continue feeding without her, as through all this I had continued lack of support from elsewhere in the family, who actively tried to stop me breastfeeding, but that's another story!