My sensei's wife trained during her first pregnancy... and the baby turned out just fine. Except that I think he's more inclined to tae Kwon Do rather than aikido at the age of 2.
On her second pregnancy as well. The advise is, don't train during the first 3 months of pregnancy. But after that should be ok. More backfalls though...
But of course, uke's gotta be more understanding I suppose. With sensei's wife, doubly so.