Obviously it is better if one starts yoga practice BEFORE pregnancy or closer to conception as the body has not experienced the radical changes (read: the release of the hormone relaxin among other things). In that way the student can learn from a body position that they are familiar with and take some risks that only they will bear, rather than their developing fetus.

Since the devlopment of a fetus requires a nurturing environment, sense, feeling, condition, et al...a yoga practice that facilitates this yin (rather than Yang) quality in the mother is most welcomed by the developing child. That sort of practice is supportive and, when taught by a skilled instructor, incredibly safe for both mommy and baby.

In fact a yoga practice now, as mentioned above, can actually assist in labor. It can obviously provide needed breath work but it can also open up the connective tissue between the ribs thus making additional room in mommy for baby's growth. From what I've heard there is nothing less comfortable than a fetus jamming into mommy's ribs for three months.

When properly instructed, the yoga practice for a pregnant student is typically broken down into trimesters. What is appropriate for mommy in the first three months is not appropriate (or may not be appropriate) in trimester three. Again, something to sniff out from the teacher one has in mind for the teaching. If they do not know that, it is best to move on to one who does, as it is rudimentary.

Additionally, there are certain things in yoga pregnant mommy should not do. Runnning and jumping are among the list. There are others but there's no point in being any longer-winded than I already am.

Short answer: Yoga is fine for the pregnant student when it is calming, nurturing, and over-engineered for safety.