My Sims delivered her baby when I wasn't playing

• SIM became pregnant.
• Switched over to another household.
• Went back to the original household.
• SIM had already delivered her baby & named it too.

Is there any way of preventing this from happening?

Yes, go into your game settings / gameplay to adjust time relative to your household.  You have options for: global/universal aging (all played and unplayed) sims in the universe; age only the active household; or to turn off aging completely.

[The new baby] has a horrible name and I can't change it.

Have you tried going back to the manage neighborhood screen, and then select the household and instead of playing the household, bring them back into CAS to modify the household members?  After one of my characters had married, her name didn't change, so that's how I changed her last name.  I had not tried doing this with the first name however, so I'm not sure this will work.  The one thing I noticed, once the SIM has been created, you can't muck with the family tree, but I'm sure that's the only absolute.

Good luck.

Pete.