OB app't

I had my first OB appointment a week ago Monday.
All went fine, nothing unusual,nothing spectacular. EDD is still marked for
Feb 7 '09.
I did ask my OB, though, about DD2 and her still breastfeeding. It was at
the very end, just as she was walking out the door.
I simply said that DD2 is still nursing, although not very much, and if this
is ok or not. Dr said she recommends DD2 stops nursing completely. I
didn't get a chance to go further or ask why or whatever.
Should she stop nursing? I do know very few people personally that have
nursed while pregnant and did the tandem nursing thing.
The fact that my OB straight up said stop nursing is why I'm asking here,
hoping someone might have a little info on it.
I know someone earlier recommended a book about tandem nursing/nursing while
PG, but I just really don't have the time or ability to sit down and read a
book. I just don't have that luxury anymore LOL
DD2 nurses seldom. Usually first thing in the morning, sometimes before nap
time, on occasion before supper, guaranteed each time she's
sad/hurt/whatever and before bed.
Rather than just cut her off completely, I've just sort of not offered. I
haven't asked her if she wants to nurse, but if she asks, we do nurse.
She's 21 months in just a couple days now. She asks for boobies and gets
them, but I no longer ask her if she wants, so I find that I offer more than
she asks. Should I just let her wean completely like that? Or should I
keep going and offering? She never turns down an offer