When I was pregnant, I was concerned about whether or not I'd know if I was in labor. After I figured that one out, I was concerned about knowing when my milk would come in. And, once I knew it was there, I was wondering if I'd know when it let down. Spoiler alert: you know. The feeling of breastfeeding letdown is distinct and unmistakable, in my experience, and feels as if someone is using an electrical current to deflate your breasts (I apologize for any mental images that may be causing you).
Letdown is in no way comfortable, but it's not necessarily uncomfortable and it's completely and totally tolerable because you know the one thing in the world that you need to happen more than anything is about to happen: your milk is going to be released so that your kid can have a meal. Hopefully your baby, or your breast pump (or a towel or something) is nearby.
However, I don't assume that my experience with breastfeeding and letdown reflects everyone else's because, like motherhood itself, breastfeeding is different for different women. Every mom and every body varies, of course, so it would make sense that we'd feel and interpret the sensations in different ways. However, I can't help but notice there's a common themes in how eight women described their own letdown: relief.
"Like the most intense pins and needles you've ever felt from your collarbone to your nipple!"
"Bear with me. There was an episode of South Park where this character Jimmy started getting embarrassing erections. Before they happened, there was this little jingle bell sound effect to let you know it was coming. I feel like letdown is the physical manifestation of that sound. It's like a pins and needles tingle that immediately precedes the flow. The worst, of course, is when you feel that 'jingle' and there's no baby on your nipple, in which case you'd better hope you're wearing nursing pads (or a thick sweater)."
"The tingling feels like when a limb falls asleep, and then relief, like when it's back to normal. Carpal Tunnel for your teets, if you will."
"It always felt, to me, like when your skin reacts to something (like the hair on the back of your neck from cold or startle), only it was happening on the inside of your breasts. It was never gradual-feeling, just all at once, and then I knew I was ready to nurse."
"So much relief. It's that Ahhhhhhhhhhhhh moment."
"For me, it feels like tingling but in a line, like through a specific vein or something. Know what I mean? Like a straight shot of tingling down."
"A twitchy electrically charged spider crawling down your breast and then complete relief."