Massage is not recommended during the first trimester for legal reasons, just to avoid blame being put on the therapist if anything does happen. Concerns about directing blood away from the growing fetus sounds like an excuse to not do a massage in the first trimester without admitting it's got anything to do with legal therapist protection rather than concern for the mother and baby.
Yes, massage can affect blood flow and lymph flow, but not enough to interfere with the growing fetus - MANY MANY MANY people receive massage in the first trimester before they realise they are pregnant, with absolutely no ill affects, if anything, decreasing the stress level of the mother helps her relax which can increase the blood flow to the areas where the blood flow is needed; many people receive massage in the first trimester after they know they are pregnant because the therapist doesn't know it's not recommended.
Would I take on a new client in the first trimester? Not a chance. The legal implications are just too much, something like 20% of pregnancies end in a miscarriage in the first trimester, often it's not even noticed it IS a miscarriage because they hadn't realised they were pregnant. Week 4 when the thyroid levels become critical is a HUGE reason for miscarriage, but often not noticed.
BUT I have massaged regular clients through their first trimester, both before and after they knew they were pregnant.
Basically my rule is the first trimester the mother IS in a more vulnerable stage of pregnancy than later on, you don't change routines, you don't do anything unusual. If she gets a massage twice a week before she is pregnant, she should get a massage twice a week even in the first trimester, because you don't want to introduce any unnecessary changes to her body, stopping massage is a change. If she gets massage ones every 3 months and wants one in week 8 I would say wait until week 12. If she got a massage supposedly every week from someone else that will no longer massage her because she was pregnant, I would NOT take her on as a new client until week 12 - you don't know what the regular massage was like, depth of pressure, areas worked etc, all therapists are different, you will be providing a different massage, it would be a change to her routine that may have an impact. I find the line "no unnecessary changes to your routine should be made in the first trimester" is sufficient for most people.
There are no legal ramifications for NOT giving someone a massage because of contraindications, and not in the first trimester is a commonly referred to contraindication, even though I doubt you will find any scientific evidence for it being a contraindication. When in doubt don't and all that.