Is it possible to choose in the Sims 2 whether your Sims have a baby boy or baby girl?

No, unfortunatly not.

Yes, actually you can. You don't have to get your sim pregnant for this. Also, the youngest age you can have this child is a toddler. Here's how:
1. CLick Shift+Ctrl+C
2. Type in, boolprop testingcheatsenabled true
3. Click on any adult sim
4. Click Spawn
5. Click Tombstone of L and D
6. When a tombstone appears, click it, and click New Toddler Girl or New Toddler Boy. You can also pick adults or teenagers.
-Gina032499

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i wish u could cuz my sim always has boys! in fact she has 5 girls and 7 boys (yh i know she has 12 kids but they in fact only 1 lives with them now them being the mum and dad) both parents had a to have want to have 10 kids! they r all in college except one (i have the university ep)
by DestinysHeart ♥

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You actually can.....
When you think that your woman or man is about to have a child
save the game and go back to the neighborhood.
then you go back into the game and let your person have the child.
If it is the gender you want keep it or save.
If its not the gender you want,
go to the neighborhood without saving and keep doing it until you
have the child you want.
hope i helped!! :)