The experience of bringing a child into the world is unique and to be cherished. I myself have never carried a child, however I have gone through all the ups and downs with my "blood" sister. (My god son is just turning 1-- we've both been there for him since day one, as his realy father isn't in the picture)
One thing I havn't seen brought into this thread is the issue of joints. As a womans pregnancy progresses her hormones gradually relax her joints. This could be a major issue with any technique that puts pressure on her joints. As with all things your wife will want to talk to her physician about any pre-existing conditions which could play a bigger rule in her pregnancy. I cannot stress enough about keeping hydrated and keeping one's blood pressure at a safe level( this rises during pregnancy making physical activities iffy for some women). I letterally had to carry dear Sissy to the car because she had overexerted her self ( not an easy task with someone a foot or so taller then me!).
IMHO given that a women is in average physical shape, she can practice Aikdo up until the last month of pregnancy. No ukemi or atemi to her belly, but she is still practising, to the level where she and baby are comfortable.
Once again, I wish you all the best in the coming months and please do not hesitate to email me with questions or concerns-- we're all nurses and midwives in my family