When I was pregnant I asked my doctor if a heating pad was safe to use on my stomach while I was pregnant and the doctor said the heating pad was just fine and safe to use on my stomach when I was pregnant.
The doctor said that the heating pad would not raise my core body temperature so the heating pad would not cause any miscarriage or other health issues with the baby.
The heating pad will also not get the baby too hot either as your stomach will protect the baby from the heat of the heating pad.
When your core body temperature increases that's when the baby could have health issues or you could have a miscarriage.
But since the heating pad does not cause your core body temperature to increase like a fever would the baby should be safe.
I used a heating pad on my stomach and back when I was pregnant and the babies were just fine and healthy and I didn't have any complications with the pregnancy either.