How about making her the matron of honor and choosing a maid of honor? You could also make her the honorary matron of honor and have an actual maid/matron of honor. I had a similar situation a few years ago with a best man who was called to military duty. It was touching to see his video tribute to his brother and to know that he was there in spirit.

You might video the ceremony for your friend so she can feel connected.

Congratulate your Maid of Honour and thank her for her help so far. Tell her you are sure she'll understand if you now call her the Matron of Honour. Ask another close friend/relative to be Maid of Honour. Make sure both know you regard the poitions as equal and that the new Maid of Honour os not "replacing" the now Matron.

I don't know that changing titles around will get you more assistance. I suspect your friends and family will give you help as their personalities, not titles, dictate.

I'm not entirely sure why you would be "sad" not to have a MoH standing with you. To miss a particular person, such as Pregnant Friend, as a reason to be sad makes sence. However I don't understand exactly how a certain set of people WITH titles would make you happier than that same set of people WITHOUT titles. It isn't really any different, so I guess go for it. However, don't then add another BM, or you will have replaced Pregnant Friend. Let the Bride's side have one less person than the Groom's. It's ok.

If I'm understanding you correctly, it's not so much about the title as it is about the idea that there is one special person who is sort of responsible for all the things that an MOH would normally be responsible for, both at the wedding and in the days leading up to it.

I think you have some options here.

You can assign one of your other bridesmaids to step in and take those duties.
You can split those duties among all your other bridesmaids.
You can ask a family member who is close but not already in the wedding to take those duties.

You can do any of the above without actually calling someone "maid of honor".

If you're worried about having an even number of bridesmaids and groomsmen, I think that your friend would understand and not feel bad that you asked another person to stand up for you.

These days, though, anything goes. I've seen weddings with lopsided attendants and it was just fine. Unless you're printing a program (I guess that's almost the norm these days?) no one will even know if you have a MOH or not, they'll just see the attendants standing up there. Even if you do have a program and just name your absent friend as MOH, again, no big deal.

The thing is, there just aren't any "rules" about these things. If you want to have two or more or no MOHs, who's going to tell you not to? It's all about what makes you comfortable as long as those attendants are all on board.

Lowspark nailed it.

Just have the people you want to have in your wedding up there with you, and don't worry so much about "titles" -- they are meaningless anyway -- even numbers, etc., etc.

I cannot believe your friend would be insulted, offended, or hurt no matter what you do, including if you name someone else maid/matron/man of honor, or two of each, for that matter. You did ask her first, after all, and she is the one who had to step down; you didn't "fire" her. I'm sure she doesn't expect to have any say in what you decide to do.

I doubt she cares much if at all what you do. Not because you and your wedding aren't important to her, but because, if memory serves, one month before her due date, she is not likely to be fixated on someone else's (even her best friend's) wedding!

She is making as big a fuss for you and your wedding as she can given her circumstances. Make sure that you make a big fuss about her pregnancy and baby, too.