I'm currently tandem nursing 3.5yo dd1 and 18mo dd2. I nursed all the way through dd2's pregnancy though we did night wean a few months into the pregnancy... I had no night supply (which meant dd1 would latch on in her sleep, there was no milk, she'd wake up and scream bloody murder) and it hurt!
This time round my body seems to be doing something similar... during the day breastfeeding feels a bit rough but generally ok (and dd2 nurses probably 5-7x a day). At night breastfeeding is still ok except during let down. And then it feels like broken glass is getting squeezed out my nipple. It is really painful for about 1-2 minutes, then goes back to just a sort of slightly uncomfortable ache. I remember this happening last time too... after a few weeks of painful letdown it became all painful and no letdown.
DH and I are thinking that we'll gently encourage dd2 to nightwean as well (we used the No Cry Sleep Solution for Toddlers and a very modified Dr Gordon plan with dd1 and helped her nightwean over the course of about 2 months).
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My vbac OB was a LLL leader for years (as well as an OB who did an internship with Ina May Gaskin) and she felt that as long as the mama didn't have a history of pre term labor or other serious medical concerns there was no "biological/medical" reason not to nurse as long as mom was eating a well balanced and complete diet with some sort of back up supplement. She explained pregnancy breastfeeding and self weaning like this...
Breastmilk is a supply/demand product... nursling nurses more, there is more milk. But this process is controled by hormones and during pregnancy the "pregnancy hormones" trump the "nursing hormones". So even if the nursling nurses more, the system can't go beyond what the pregnancy hormones will allow.
In early pregnancy the milk volume tends to drop, and the taste/composition changes (saltier, more minerals). Some nurslings just don't like the new flavor and the extra work required to get as much as they want, and they self wean.
By the end of the pregnancy the taste/composition changes again (begins shifting to the newborn milk, so thicker and fattier) and nurslings who kept going despite the earlier changes may decide to wean now. (be prepared for an older nursling to chunk up a bit when the new babe arrives... they're essentially moving from skim milk to ice cream

)
For some reason that last bit never occured to me and I was actually a bit worried when dd1 gained a whole pound right after dd2 arrived... I mean she was/is totally healthy and active and in love with her little sister but all of a sudden she had "baby rolls" of her own!