I don't think I've gone 12 hours before but I routinely go 9-10 hours overnight without adversely impacting my supply. If I get a later start to my pumping in the morning, I'll often drop a session for that day so I'm not trying to squeeze in my normal ppd, since it seems like my body produces the same amount either way.
I wouldn't chance it, personally. I have heard from pro EPers that it's necessary to maintain a MOTN pump session, I don't completely agree with that as long as you empty often enough during the day. That said, I would not go more than 8-9hrs without pumping overnight. The way i look at it, you're going to do it one of two ways: 1. Pump at even intervals throughout the 24hr period, say every 4,5,6 hrs in order to minimize the total number of sessions. In this case you will have to get up at night at some point. Or 2. Pump more frequently during the day, to mimic baby's feeding pattern, esp if the baby is STTN. Then make sure you pump right before bed (say 9-10pm) and get a good long stretch of sleep and pump first thing in the am (say 6am).
I always did it the 2nd way and had no supply issues. My last pump of the day was at 9pm and then after my LO started sttn I set my alarm clock to 6am to pump. So i was going 9hrs without a supply hit.
This one instance, I seriously doubt you have damaged your supply. I would say, if you notice a difference, power point for a few days and that'll get you back on track. I took steroids about two weeks ago that really shot my supply so I discontinued taking them and it's back up to "normal." I vaguely remember very, very early in my EP journey, my husband was being nice and let me sleep in and I about how a heart attack. We've been EPing since about a month with no supplementing so I really don't think one time is going to hurt anything.
I've been going 9-10 hours overnight since DD was 4 months old. It's different for everyone, but I still pumped what I needed in my 5 pumps during the day. I've been 4 PPD since she was 5 1/2 months and now at 7 1/2 months I'm 3 PPD and still consistent with supply.
I agree with PP that going 9-10 hours overnight on a regular basis is probably okay assuming you are still producing enough during the day but I wouldn't routinely go for 12 hours overnight at 3 months old- weekends maybe, but not everyday. Maybe when your LO is older.
ETA: doing this every once and a while is okay but may lead to clogged ducts.
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