I'm trying to do my internet research like a good girl, but seriously, there isn't much out there on the topic of "martial arts" + "pregnant."
I found this: http://www.pregnancytoday.com/articles/healthy-safe-pregnancy/martial-arts-for-the-mom-to-be-769/ It mostly talks about tai chi and kick boxing, but the principles seem to be the same.
http://www.pregnancy.org/question/what-precautions-continue-kickboxing. This is specifically about kick boxing, but it sounds like the same stuff as taekwondo kicking.
http://www.centrical.com/blog/8862111798780185820/martial-arts-and-pregnancy.html. I find this one the most comprehensive and yet, it's the one written by a guy. Don't pregnant ladies ever write about their training?
http://www.defend.net/deluxeforums/womens-counter-offensive-discussion-forum/27243-mt-pregnancy-conditioning-bag-work-etc.html. This one wins points for talking about how practicing falls and rolls can help when you fall in real life. Cuz us pregnant ladies have no balance. I'm still waiting for the day I actually fall down for the first time.
http://www.pocatelloparents.com/2009/01/26/tae-kwon-do-during-pregnancy. Yay! Finally, I find evidence that pregnant ladies have successfully gone to taekwondo. She went to class on a Thursday, gave birth the following Saturday. It sounds like her training was all "no contact," which is not what my school does, but still.
http://www.youtube.com/watch?v=WkQcJVhvsoM. Favorite thing I found today. A woman who is nine-months pregnant does koryo, the black belt form. I love that at the end she says the baby is kicking now.