Why are pregnant women NOT supposed to use the massage feature on heating pads, chairs, beds, or other items?

Why are pregnant women NOT supposed to use the massage feature on heating pads, chairs, beds, or other items?
OK I have had 3 kids and I have never heard that before. I have heard that you shouldn't stay in water or a tub very long once you reach 8 months. I didn't listen to any of it. I have problems with my back and my doc. told me to use heating pads and to change my bed as much as I needed to. It was that or prescription drugs.
I understand about heating pads and hot tubs changing body temperature and possibly harming the baby. What is dangerous about using an electric massage pad during pregnancy?

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