I was someone who struggled with clogged ducts and mastitis. Clogged ducts are annoying and painful. I can also tell you mastitis is likely the worst thing ever. No mama should to have to deal with it if you don't need to. After battling the blocks a couple times, I discovered that I needed to prevent my ducts from clogging in the first place. I did that by altering my wardrobe. These two products were such life savers for me that I carry both in my store. The Momzelle Seamless Nursing Bra and the Glamourmom Nursing Tanks. I found both these products to be nice and stretchy and not restrictive on my breast tissue at all.
Underwires were long forgotten during my breastfeeding days. I didn't even go close to them. Even bras that were a bit too tight and didn't have any give or flexibility in the material would take me down. I had one underwire-free nursing bra in particular that got me many times before I finally threw it out. It was a moulded cup bra; one I had bought during pregnancy at a big box store when I had no idea what my breasts would be like postpartum. You live and learn.
Truthfully I rarely wore an actual bra while nursing. I almost always wore a Glamourmom Tank instead. The Glamourmom Tank was like nothing I had ever seen before. It has a built-in bra with a full sized band and clasp at the back. The tank also has great length - making it the perfect layering piece. For me they were perfect for hot summer days and ideal for layering in the winter.
I didn't get to know the Momzelle Seamless Nursing bra until later in my nursing journey. It was love at first wear though. It is the most comfortable bra I have ever owned. I do find you trade some support for comfort in this bra. But if you just need something to hold your nursing pads in place and not cause you issues if baby decides to take a 4-hour nap, this bra is perfect for you.
As for why some women encounter clogged ducts on their breastfeeding journey and others don't at all? I'm not actually sure why, but I imagine anatomy has something to do with it - like how some people are more prone to ear or sinus infections. I also found that stress tended to play a part in it for me - I was much more likely to get a clogged duct if I was feeling tense about something. Makes sense, as we tend to feel stress and tension all throughout our bodies, then if you throw on the wrong bra with that it can head sideways quickly.