Yes, that "no massage in the first trimester" is more for the benefit of the therapist than the client - higher risk of miscarriage, so generally recommended not to.
As stated before, a lot of women don't even know they are pregnant until 6-8 weeks. With new clients that ring up and say they are 8 weeks, I do usually ask them to wait another month - though I have very few enquiries for pregnancy massage that early in the pregnancy.
Existing clients that tell me they are pregnant at 4 weeks, I will happily continue to work on them through the first trimester.
One of my regulars was still still feeding her baby when she fell pregnant again, so she didn't really notice she was pregnant. Her neck, shoulders, upper back were all a tight from feeding and lugging huge baby around, and we'd been using heat packs on her neck, heat packs on her shoulders, deep work on the shoulders in the area to avoid during pregnancy, plus working the heels during a pedicure/mini reflexology session. I'd used a low concentrate of some "never to be used in pregnany" essential oils (uterine tonics), but it was a very low concentration as she was still feeding the other baby. We'd even been using heat packs on the belly, but I hadn't done that for 6 weeks before finding out she was pregnant - not sure if that was just me intuatively forgetting to do it. Anyway, pretty much everything else would've been a no no for a pregnancy massage, and the pregnancy continued on quite happily. She was about 9 weeks pregnant when she eventually noticed. Her ob told her to stop getting massages immediately so she cancelled her next appointment, once she was past 13 weeks the ob said she could have a massage again. She's 7.5 months now and all going along smoothly.