In general most massage is entirely safe during pregnancy, but there are some general considerations to keep in mind. There is a huge difference between the full training in Chinese Medicine of which tuina (medical massage) is a part and our US based massage training. The biggest difference is knowing more about how the body functions in Chinese Medicine terms which then helps you to think through problems better than knowing muscle/anatomy and then possibly a few common acupressure points here and there.
Part of this becomes crucial when you need to decide which patients might be more likely than others to have an issue (i.e. which have the basic underlying Chinese Medicine diagnostic pattern that puts them in the category to be more prone to miscarriage). And it's underlying this issue is why there are no general rules (that and our current legal environment).
Because massage therapists do not have this theoretical training, massage is probably best avoided until about week 15. After that time except in less common circumstances, the baby is well established and not much can go wrong besides obviously wrong techniques - very vigorous work on the low back, deep visceral abdominal massage - things that would be wrong just from a common sense standpoint, not just a training standpoint.
Now there are a range of contraindicated pregnancy acupuncture points which should be looked through, but the general idea is you avoid strong circulation and directional energy in a downward and/or outward direction (which some points do regardless of technique in general terms). And you do this even more so in any patient that shows signs of qi, blood and/or yang deficiency (which would generally be strong signs of fatigue, coldness, etc.).
Now there is acupuncture for labor induction (among many other treatments) so there is a strong clinical basis for the points to move a women to labor - it's used quite often. That said, it generally won't do much unless the woman is already heading that way. That is it works more with the body than forcing it to do something it wasn't going to.
Now all that said, massage can be quite useful for a range of conditions during pregnancy, but some common sense and a basic idea of what to avoid are crucial. To say you couldn't cause an issue with a woman particularly in early term or during an unstable pregnancy would be unethical. But the odds of this with some common sense and a general avoiding of downward and outward points and movements are much lower and the benefits it would offer make it a very viable modality to use during pregnancy.