MM I’ve followed your pregancy and troubles as well, and have bitten my tounge several times on advicing you to leave. Your and your baby’s health is parmount.
As someone who receives letters of resignation, my recommendation echos the simply, but brief. Thank them for the good, skip the bad and don’t offer details of why you are leaving. If they ask, you can say as much or as little as you want, but there’s no merit in including the reasons in a written document.
In general, if you find another job before you leave, then you can always say that you’ve got a better job offer, which is an easy out.
In your case, less said, the better. Don’t bring up department politics or the current situation (which might, indeed, be temporary.) If you want to say that it’s for health reasons because of the pregnancy, that would probably be enough for any exit interview. Remember to keep things friendly – think in the back of your mind that, when you decide to return to research, this place could be very different and you might want to apply there. Academic research departments at universities can (and do) change over time as grant money comes and goes.
I guess if you’re quitting because you’ve won the lottery and never need to work again, no holds are barred.
My wife’s friend left a job that he disliked.
The first line of his resignation letter read “It is with measured regret…”
Thank you all for the suggestions and good vibes. 
I’m writing a draft letter of resignation. At the end of this month, I’ll be going on vacation. (A week in Yosemite! Yeah!) The plan is to quit after our trip and after I’ve used up some of the sick time I’ve accumulated. (There’s a couple hundred hours of it. One big downside of this job: you must be in the lab or all hell will break lose. I was once out for a week with viral bronchitis and a lovely secondary stomach infection - my co-workers called me several times each day.)
Finally telling Mouse_Spouse that I was miserable at work and wanted to leave was very difficult. After we looked at our budget and I saw that we would be OK without my salary, it was a relief. I cried. It was relaxing. It was cathartic. It was interupped by one of the cats tapping me on the head and meowing “Feed me, bitch!” (Retired alley cats don’t have good manners.)
Next order of business: figure out a new online handle since I’m going to be de-moused.
May I respectfully suggest Cat_Food_Bitch?