Baby pulling away and crying then relatching was a sign of pregnancy with my 2nd and 4th (1st and 2nd 12 months apart, 3rd and 4th 13 months apart) and then again though didn't happen as often when I found out we are expecting our 5th (this little one will be 20 months from 4th).
I did continue to breastfeed till my third was 10 months (so 3 months before delivery) and then she self weaned. This time since my baby was already on solids when I conceived she has cut back to more to nursing for comfort, prior to naps, and nighttime nursing versus all day long but we're still going pretty strong, I'm 4 months along and she's now 14 months old.
If you are expecting you can keep nursing and should check out the Tandem Nursing thread
In my experience I usually have decreased supply mid second trimester then my milk comes back a few weeks later. Though I haven't nursed all the way through birth yet many women have and I'm letting our 4th baby self wean as I did my 2nd and 3rd so she could nurse till her sibling arrives and beyond we'll see.