I have a 4-week old baby girl. I had been giving her breastmilk exclusively until last week when she started eating like a pig (4-5 oz every 3-4 hours). I started supplementing her with formula last week because she has a twin sister and I'm not able to produce enough breastmilk for both of them. So, I started giving her 2 oz of Similac Advance in each of her bottles (the rest was breastmilk). Well, a day or two after I started doing this, she stopped having regular bowel movements (after each feeding, like she normally did), and only started going once a day. I did some research and found that it is normal for an infant to make a BM only once a day or once every couple of days, so I wasn't too worried about that. But, when she makes a BM, it's very loose and a darker brownish/orange color (which can be from the formula). They have a doctor's visit tomorrow, so I'm going to ask about it then, but I'm just wondering if the loose stool is just from the change of mixing formula with the breastmilk, or could it be a sign of something else? I don't think she's dehydrated at all (she has 5-6 wet diapers per day). She is otherwise very healthy. She's not gassy, or unusually fussy/colicky, and she eats and sleeps on a good schedule. Any thought? Thanks in advance.

She's fine. Poops will change a ton in the first year. You'll notice lots of changes with any food variance, in this case the formula. You'll also notice changes during growth phases as well. Infant poops are always really wet, they don't end up pastey or looking like "grown up" poop until they start eating regular food.

Normally, if a tiny baby has loose poop from illness, you'll also notice a change in the odor, it will smell nastier.

none of my kids could tolerate Similac advance, Isaiah had really runny green stool so dr told me to change his formula, so I did

Formula tolerance is a good point Valerie made. Some just don't agree with some babies. I've never had any luck with any of the Similac brands. Good Start is the one I favor. But, I think a lot of Moms change formulas too quickly thinking it will help with runny poop, gas, whatever and it can exacerbate the problem. Any food change, liquid or solid, takes at least 5 days to see if it will work out or not.

Do whatever it takes to make you feel comfortable, you have enough to worry about without worrying about the poops too.

I was thinking about changing formula, but I was going to try to wait it out a little bit to see if things got better. I don't want to keep changing around things in her diet. Her poor little belly is probably going through enough right now with me just introducing the formula.