My 3-week-old baby has mucus/blood-tinged stools twice a day. Is this normal? What could be causing it?
I'm very glad you asked this question, as there can be a variety of reasons why a young babies might have blood in their stools—some of which are relatively harmless and others which require more immediate attention. Regardless of the severity or underlying cause, technically speaking any blood in a baby's stool is not considered "normal" and any baby who has blood in his or her stools should be evaluated by a pediatrician to help determine the cause and what needs to be done to further assess and remedy the situation.
Evaluating a baby with bloody stools should always include asking what else is going on with the baby. How is she eating? Sleeping? Growing? Is she fussy and/or seem to be in pain? Is there any vomiting? It will not only be helpful to provide your pediatrician with this sort of general information about how your baby is doing overall and if she has any other symptoms of concern, but also to bring in a stool sample for your doctor to see/test (you can simply fold a dirty diaper over on itself to keep the sample well contained and then put it in a plastic bag to bring with you to the doctor's office).
While your question is one that can only be answered specifically by an health professional who has had a chance to examine your baby, see a stool sample, and have you answer questions like the ones above, in general, there are several possible causes of blood in the stool of a 3-week-old.
When babies are first born, anything from swallowing their mother's blood during delivery or a small break in the skin around the rectum (anorectal fissure), to problems with the intestines and/or blood clotting issues can cause there to be bloody in the stools. As infants get a bit older (1 to 3 months), intestinal abnormalities or problems are certainly still important to consider and rule out, but it becomes increasingly useful to also consider a milk and/or soy protein intolerance.
Simply put, milk-soy protein intolerance (MSPI, or milk- or soy-induced colitis) is a not uncommon condition (estimated to be anywhere from 1 to 10 percent of babies) where the intestinal lining becomes irritated by proteins in the breast milk and/or formula, causing some bleeding as a result. In the case of MSPI, babies typically have loose stools, often with visible blood, but otherwise don't have any other problems or concerning symptoms. Once the intolerance is identified (often when blood in the stools is noticed by a parent and brought to a pediatrician's attention), babies typically do very well by simply adjusting their breastfeeding mom's diet (to eliminate the irritating proteins) or, for formula-fed infants, changing to a special type of formula. The good news is that milk- or soy-protein intolerance fortunately tends gets better during the first year, so that children end up being able to eat a normal, unrestricted diet by the time they reach toddlerhood (if not well before).