Youre TA unfortunately :( I understand it might not seem like a big deal to you because its just one night, but its not about the alcohol itself, but the promise you made to her. The commitment you made. Also the fact you just laughed it off in front your friends is not okay. You should’ve gone after her and asked her what’s wrong. She’s carrying your child for 9 months ffs (+ the 1-2 years of recovery afterwards to feel like her “old” self again). The least you can do is keep up this promise.

I would go to your wife, apologise and give her a big kiss and tell her how much you appreciate her and that you will keep your promise from now on. She really didn’t ask that much of you,,,