What you are pumping is not always a good indication of your milk supply.
It certainly could be decreasing, but is your 9 month old nursing less and/or eating more solids. Those could both encourage a natural decrease for your milk supply.
I've heard lots of my friends say they stopped nursing number 2 because of a milk supply issue while pregnant.
I nursed my son successfully while pregnant and now tandem nurse both he (2 1/2 years old) and his 7 month old brother.
I would encourage you continuing to nurse both for the benefit to your nursling and for the easier transition that tandem nursing provides. We certainly had some jealousy issues, but not many.
Consult a midwife or lactation consultant and see if you can take some supplements while pregnant that will help your milk supply....like Mother's Milk Tea, etc.
Good luck and congrats mama!