K.W.
J.,
It is normal for your 8 month old to be able to get everything he needs in 6-8 minutes, so he is still nursing fine. As your pregnancy progresses however your milk supply will dwindle away. So whether he likes it or not you may wean due to pregnancy. I am very impressed that you have such a great supply at 20 weeks. You certainly won't have any problems feeding the next one either.
K. @ The Nesting place