Just because you are a quilter and you made baby quilts for the first two babies - there is NO RULE which says you MUST make one for the upcoming child.
You're right, she could become a baby factory (no insult or pun intended) - we have two very close friends who believe in large families, one just had their 11th baby and the second family is working on their 9th. "OMG!!!!!"
If you truly don't feel inclined to make another quilt, you can always purchase a gift of some other kind (I wouldn't buy a quilt made by someone else as they could take offence YOU didn't make it).