Medieval Catholic iconography is full of symbolic images of Mary feeding baby Jesus or offering her bare breast. Images of breastfeeding also show up as symbolism in non-specifically religious contexts as in Carità("Charity") by a 14th century Italian artist. According to Margaret Miles (Secularization of the Breast 1350-1750), the breast became more sexualized in Western Europe in the 16th century, and images of Mary breastfeeding Jesus became less common.

While religious treatment doesn't necessarily perfectly coincide with day-to-day treatment, the prevalence of both the image itself and the symbolism of breastfeeding as charity/mercy (as well as the more prosaic issues of practicality and the relatively non-private lifestyle of most medieval people) leads me to suspect breastfeeding was probably commonplace and not considered taboo. While it addresses a later time period (17th-18th century) you might find this article on exposed breasts in clothing interesting.

A straight forward question, sure, but one which is not being actively researched as far as I can tell.

The nearest I was able to get was this article by Sara Matthews Greico, published by UNICEF in 1991, which examines the theory, medical history, and societal custom of breastfeeding over about a 400 year period of 1400 to 1800. Disclaimer: I had problems with some of the author's claims, especially concerning the use of corsetry affecting female physiology to such a degree as to make inverted nipples a "widespread" problem that impacted women's ability to breastfeed between the sixteenth and eighteenth centuries. As a clothing historian, when someone throws that sort of thing out there without any sources to back it up, it raises all kinds of credibility red flags. Near as I could tell, since Greico did not bother to cite her source for that claim, she pulled it from this 1913 publication by the American Journal of Obstetrics & Diseases of Women, which is hardly reliable, let alone dealing with the scope of her research.

But I digress... While there's some serious scholarship done on the topic of breastfeeding in general societal terms during this period (this, this, and this all look promising), it doesn't appear that any authors have been interested in looking into where breastfeeding took place, as opposed to how breastfeeding was expected to be done culturally. I'll keep looking for more answers, because now you've got me curious, but right now I'm not coming up with a concise or satisfactory answer.

Hi, coming in for part two of this question. I've asked around some of my academic contacts and pretty much came up with the same response of "Now that you mention it, not a lot of research has been done on where a woman was supposed to breastfeed in the middle ages..."

So, my next stop is to look at art, since I am an art historian by trade. I dug up the following images. Since representational art, particularly of commoners, didn't get going until the fifteenth century, the bulk of these images come from the sixteenth century.

• I was expecting to find more instances of breastfeeding in Bruegel the Younger's paintings, considering the sheer amount of public urination he includes, but the only instances I could find off hand were St. Kermis and the Dance Around the Maypole in the lower right corner, Visit to the Farm, which shows a crowded interior scene that almost looks public, and this version of The Peasant Wedding which might possibly show a nursing woman (unclear if she's just holding it or nursing it). Of course, Bruegel being Bruegel, you're up against all kinds of interpretations of what the activities are meant to signify, and I'm not as well versed in this area so I can't do more than speculate.

• In Joris Hoefnagel's epic painting Fete at Bermondsey, 1569, there, right smack dab in the middle of the action, is a woman breastfeeding (much to the interest of the man beside her). Here's a closeup of the couple in question.

• For an upperclass depiction, there's the version of Gabrielle d'Estrees and her sister in the bath that shows a wet nurse sharing the center focus of the painting. It's interesting to note that Gabrielle and her sister are covered in this particular version of the infamous double portrait, so that the focus is actually on the wet nurse and the baby. And obviously, Gabrielle's beauty is unquestionably preserved by not breastfeeding, in contrast to the wet nurse.

Here's my conjecture based on the survey of art here and elsewhere... Public breastfeeding wasn't common, but it wasn't totally uncommon. It was far more likely to be either a wet nurse or lower station woman who could not afford a wet nurse depicted feeding publicly, while wet nurses who served high station women appear to be depicted indoors only. High station women, of course, were not typically breast feeding their offspring (atypical instances do crop up, such as Anne Boleyn who caused a lot of commentary to be written by her decision to breastfeed her daughter in 1533).

I'm going out on a limb here to say that unless you were of a lower station, you breastfed indoors and not publicly. If you were of a high enough station to afford a wet nurse, you didn't breastfeed at all.