At one point, back in my "wild" college days (they were really quite tame), I was into body mods. More specifically, I was into guys with body mods. The crazier the piercing, the more I was into him. I digress. I had a few modifications myself: standard earlobe piercings, a small stud in my nostril, and my nipples. I had them for about 6 years and took them out just before we started trying to conceive.
I thought I did my research prior to getting them at 20: I made sure they wouldn't interfere with breastfeeding later in life. I checked BME since I didn't know about Kellymom at the time (side note: wow, I've come a long way), and even double checked with the piercer right before I got pierced.
My parents knew about them and while they weren't thrilled, they were thankful that it was a hidden piercing. I distinctly remember having dinner with my parents and sister soon after getting them done. My dad told me that milk would come out of the piercing holes when I breastfed my baby. He said it would be like a fountain. I laughed and laughed at the mental image until my stomach hurt. Guess who had the last laugh? Hint: it wasn't me.
I remember one of the first times I let down while pumping. Milk came out of the front of the nipple (as you would expect), but it also started squirting out of the sides where the curved barbell piercing was. My dad was right!
A Giambologna lactating mermaid from The Neptune Fountain (Fontana di Nettuno) in Bologna, Italy
Image from Peaceful Parenting
In general, I don't think the piercings have made a huge impact on breastfeeding. I have some scar tissue on my right nipple - it may be why E doesn't like to nurse on the right side, which has led to less milk production on that breast. There are many women who have not been as lucky. While I knew I wanted to breastfeed my babies at 20, I didn't have enough foresight to really think about the consequences of putting two additional holes into my body. That being said, I'm not sure I would recommend them to young women who want to get them done.