I'm not finding much on this problem. But if either of your Sims have been married before, this may cause them not to be able to get married again.
If that's not the case. You can try a couple of things to see if they help.
1. Reset Town, Individual Sims and/or Evict Household
http://www.carls-sims-4-guide.com/forum/index.php/topic,20732.msg401812.html#msg4018122. Go back to a previous save-game.
Go to your Sims 3 folder:
C:\Users\<Username>\Documents\Electronic Arts\The Sims 3\Saves
Look for a .backup or other previous save-game file with a similar name to what you've been naming your saves with and rename it something like: Save-game.sims3 (for example) Load the save to see if it helps allow marriage and baby.
3. Wait it out a couple more in-game days. She might surprise you with a baby.
Hope this helps,
MrsFlynn