Re: This is getting extremely frustrating!
Assuming you know he is eating plenty (based on mommal's comment) and YOU are not extremely unhealthy to the point of malnutrition, I would venture to say this is NOT a milk issue and your docs need to start looking other places for the problem. It's possible they want you to try formula just to rule out an issue with your milk, but the comment about 'skim milk' makes me doubt that is the case and even if it is, IMO they are going backwards if that's their plan. They should rule out other issues BEFORE asking you to jeopardize your nursing relationship because in most cases, if there is a problem, it is all the more important your baby continue to get your breastmilk.
Excuse me if you've posted this elsewhere but is your baby actually underweight or just not gaining as quickly as your doc would like? How much does he weight now and how old is he? Is he behind developmentally? Unhappy or lethargic? Any other health issues or symptoms of a problem? Does he poop/pee normally? Has he been ill frequently?
Have they done any blood tests to see if he is actually undernourished? A metabolic panel is what I believe it's called- they look at various protein levels and all kinds of other indicators of nutrition. Any stool samples to rule out a parasitic infection or GI problems? Any hormonal testing to make sure there isn't a growth problem?
Sorry for all the questions, I know it's frustrating. These are some of the things they looked at with my son so I'm just throwing some options out there.
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