I remember some old movie with Ryan O’Neal and Ali McGraw where she referred to the kid as Bozo. I have to assume she meant Bozo the Clown, as opposed to the tribe of African Bozos.
Me, I’d be more inclined to name it STD. I mean, children are the most prevalent form of Sexually Transmitted Diseases.
Cletus the Fetus is standard in my family.
We alternated. Sometimes our unborn kids were The Alien; other times they were Excretia or Secretia. What? We were trying to figure out offensive names!
To my mom, both were Little Peanut.
Either parasite or fetus. No cutesie stuff for us.
Plus it was kind of fun to see the horrified look on some peoples faces when I talked about my parasite.
My wife and I just called each fetus “Junior”, even though they were all girls.
Just wait until he starts bouncing a baseball around the walls of your uterus.
Love Story It was a good name for a hockey player, like ‘Bruiser’ or somesuch.
We never nicknamed our, but I have a great niece and great nephew who were Poppy and Cletus, respectively.
A friend has a granddaughter who was Tyke, and a nephew who was Nemo. I sort of wish the latter had stuck.
I’ve heard Cletus and Uternaut.
My niece-in-law, who’s currently pregnant, calls hers “McNugget.”
Peter Framptonette. (We knew we were having a girl.)
Ippy, short for ipecac, due to the effect she had on my wife for the first trimester.
Never been preggers myself, but I used to have a co-worker who referred to “Thumper.” In the later stages. When he kicked!
We referred to our daughter as The Alien because she was very active and her movements against my belly looked like the alien from the movie trying to break out. I was sure she was going to be a gymnast. I was wrong…
Our son was Squirt, which came from Finding Nemo (he’s the small turtle that Marlin meets), which was a result of my wife’s nickname for me. She calls me Squishy, from Dory’s line to the small jellyfish. We didn’t call him Squirt quite as much after we found out that he was a boy because we had already picked out names, but he still gets called by that name every once in a while.
Our second was going to be Spud, but now we’re going the adoption route for #2, and we usually refer to our second as either #2 or Spud.
We are calling the current fellah The Noob.
Whelp, the one I miscarried in January we had been calling The Sprog. When we told people about the miscarriage, we either said “I had a miscarriage.” or “It didn’t take.” Didn’t seem that tacky to us.
“Crevette” which is French for ‘shrimp’. It seems to be pretty common amongst my friends and family.
Patsy, Eddie, and Saffie (from Absolutely Fabulous)
Bubba, even though I knew she was a girl. I still call her Bubs sometimes, as well as Dribble Monster, Little Miss Farty Pants, “The Child”, Snuggles, and very occasionally Dakota 
I called mine “Cancer”. :o
When he was born, I thought he looked like a turtle, so I called him “Turtle”.
After awhile, he became “Jude” to him but not in company and then “Judah” all of the time and now he’s “Yehudah” unless he’s upset. Then it’s “Hey, Jude.” If I’m upset, it’s I SWEAR TO GOD I’M GOING TO SELL YOU ON CRAIGSLIST!
He’s only called Turtle when he’s asleep and can’t hear me.
(I also called my little brother “Squirt” like the Hershey’s kind. When he got older, it was “Atilla.”)
Tumor
We called ours Peanut, as did the neighbours across the road who were 3 months further along. My sister’s bub was Bean, and my friend had Kickie. (2 of us in Australia, 2 in New Zealand for the poster who said this doesn’t happen in Australia).