Hi,
Sometimes during pregnancy because of hormonal changes blister like ulcers might be seen.
If it doesn't pain then don't worry.
It might be because of
vitamin deficiency. Please make sure with your gynaecologist that you are taking vitamin supplement esp. Vit. B complex.
Hope this helps. Thanks for choosing practo! Regards Dr. Ravin Shah
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Health Tips
If pain in that blister visit dentist