I'm currently expecting my first, and attended my first birth 7 years ago. I really don't think I could have doula'd in my first trimester. I was beyond exhausted and nauseous. Fortunately I didn't have any clients due then.
I did attend births early in my second trimester, and didn't find them much different than doula'ing while not pregnant.
Now that I'm into my third trimester, I find that I need to sit more, eat more, and pee more than I typically do at a birth, and it takes me longer to recover physically. Granted, I had three births in three weeks, (which is busy for me; I also work a part-time job three mornings a week)
#1 was a stillbirth (that I knew was going to be a stillbirth); #2 was a speedy lovely homebirth; #3 was a teen birth turned Stat Cesarean with a baby needing resuscitation & eventual transfer to a higher care facility. March has been a very intense birthing month, and I'm hoping that my final client birth will be smooth. I don't want to be so full of other women's birth experiences that I don't have room for my own lessons... if that makes sense?? If the timing works out, I may even re-arrange my work schedule a little so I can spend a day at local retreat centre and have breathing space for myself. I'm working my p/t job right up until I give birth, so there won't be any built-in days off.
I am now 33+5, and have one birth left to go. I hadn't planned to take on clients so late in my pregnancy, but this client (an acquaintance/ friend) contacted me right after she pee'd on the stick, and hired me in the 5 weeks between when she got pregnant, and when I got pregnant. She is planning a home birth, and has the same midwife that I do (who is also pregnant - due in July). I do hope that this final client goes sooner than later though. If she goes all the way to 42 weeks, I'll be 37 weeks pregnant.
I have felt great from 2nd trimester onward, and DH is uber-supportive, so I've basically just carried on with my usual life and routine. Maybe it's an occupational hazard, but this whole pregnancy thing feels pretty normal to me; even if it is my first.
All that being said, only you know your comfort levels. My biggest factor is determining whether to take on clients while pregnant was finances (bad as that sounds). Assuming I was physically & emotionally up to it, and knowing that normally I only have 6-8 clients in a year anyway, I wanted to take as many clients as came my way, because we'll be without my p/t job income, and without my doula income, for a few months after the baby is born. Turns out I'll have had 7 births in 5 months, all while pregnant - my busiest stretch of doula work ever. Of course, 3 of those were volunteer births for teens, so the financial goal wasn't exactly met....
The big question for me now, is how soon AFTER baby I plan to go back to birth doula work.... if I go back at all!