If you do HRT while she's pregnant, you are both going to be very hormonal and moody. Not even joking. I don't know what it's like on the MTF side of things, but for the guys, starting testosterone turns some people into emotional teenagers again. Is it similar the other way around?
I have been pregnant though, and mother nature can throw so many curve balls that you aren't expecting. What if she gets gestational diabetes, or has some other complication (god forbid, but most are easy to treat with modern technology)? Plus, there are all these worries that the doctors/nurses/experts throw at pregnant ladies, urging you to call your doctor if you feel this or that sensation, but also giving you conflicting advice on what's bad and what is normal. I don't know your wife's normal temperament, but these things can be hard for many pregnant women to deal with, and she might need support from someone who is able to tolerate her moods. And if she does become like that, she won't really be in a position to support you if you need it, either.
After the baby is delivered, things don't go back to normal right away. It will take her body time to heal and readjust itself to no longer having to support another human. If she breastfeeds, that will bring about another set of bodily changes for her. I'm sure you could transition during this time just as well as any other time, it's just that this seems to be a phase of baby-making that many people forget to consider, so I wanted to bring it up. If the pregnancy goes perfectly well and her moods/personality remain relatively the same as normal, then in comparison she will probably be at her weakest in the few days after giving birth.
I personally ended up with postpartum depression, and I did need somebody more stable than me who I could lean on every once in awhile during my treatment. (It was WEIRD, by the way. I was not actually upset, I would just randomly start crying, or have foreign, disturbing thoughts come to my head out of nowhere.)
I personally have this gung-ho outlook on transitioning, go for it full-throttle no matter what anybody else says, but in this case I strongly caution you to only do this if you are both completely willing to deal with the emotional pressure together. It's not even just about the hormones; you will be adding two very dramatic changes to your lives at the same time! On the good side, at least they will be two wonderful changes.
I think it's also worth discussing with her if she would like the time she's pregnant and healing/nursing to be a special time where the focus is the new baby? Some people are sentimental about that. If she would like that, and you end up agreeing to it, I hope she will then extend to you the same courtesy of having special you-time while you initiate HRT.