What I see is a difference in understanding of the agreement. She seems to think any addition free or not was not going to happen. What your understanding of the agreement is at this point is irrelevant. If you go back on your word (as she sees it) you will loose trust and integrity in her eyes that is not easily gained back.
Unfortunately miscommunication is a very common occurrence in relationships. No matter how alike you may or may not be, you have different perspectives due to different life experiences and vew things differently. Mix hormones in that (pregnant or not) and you have a recipe for chaos.
What I am hearing is that your wife has one focus right now, the baby she is carrying. Because she is carrying it she has a constant reminder of this. She will not understand how you can think of anything else but the baby and will likely get offended if she thinks you focus is on anything else. Essentially she needs constant reassurance that you are only concerned with taking care of her and your new addition. Simply talking about anything but this will make her question that and will lead to a fight.
I have only been married for 13 years and only have 2 kids (8 and 10) so I have some experience but not as much as some. I am lucky to have come from a family where my parents were never divorced so I like to think I have good examples to learn from. Then again, this is a forum and we are all strangers so take my opinion with a grain of salt.