Hey V.,
I am laughing at your question as we went through this with our 2 yr old waiting on the arrival of our new baby. In hindsight, I'm glad we told her early. She benefited from learning so much and now has better patience for things.
What I did: I played baby dolls with her. We watched a good bit of baby story, adoption story, special delivery and deliver me. I would point out the doctors, the mommy with the big belly and the daddy. Then later as my due date neared we would discuss the umbilical cord, that babies are covered in white stuff. That some mommy's can't have a baby and so another mommy shares her baby in regards to adopting. I kept it simple,adding some information every so often. Nearer to delivery, I" pointed out the monitors, the wires all over the mommys, etc.
But to field that constant inquiry you are asking about... I would just say "God is still growing the baby in mommy's belly." And if she became more curious or frustrated, I would tell her if the baby came before due or finished growing, she would be too small and sick.
I also took her to my doctor appts. I would tell her we were going to see the baby on the special t.v. Or that we were going to listen to the baby's heartbeat and measure my belly. My doctor's office is attached to the hospital I was to be delivering at. So I would take my toddler for lunch with me in the hospital and explain that mommy would be staying in the hospital for a few days when the baby was ready.
To keep to my theme, as my due date neared, I would tell her an extra bedtime story. The story was about her baby sister being born. Initially it included mommy going to hospital, the baby being born and her seeing her sister. Then I added in how she would spend the night with her aunt when mommy went to the hospital.
All the easy prep work did wonders. My 2 1/2 yr old was like a pro. She would tell people "that God wasn't finished growing the baby" and all about "the UN-Bill-cull cord" and even that "the baby comes out your girly spot."
I know you are early in pregnancy, congrats on the soon to be second blessing. Many prayers that everything go as planned. And what a double blessing to share the early news with your little one, both for educating and for including her. It should make the transition and addition more pleasant.
Best Wishes,
-MB