Can fat underarms (armpit) be related to pregnancy?

Yes. Breast tissue extends all the way into your armpits. Sometimes with the hormone increases of pregnancy, breast tissue swells. Even the breast tissue under the armpit swells.
Yes. Fattty tissue can acculumate in the arm pitts simply from weight gain alone. The horrmones of pregnancy can also cause swelling of excess breast tissue located in the arm pit. If thhe swelling is only on one side then see your doctor. Lipomas are collections of fatty tissue that can be surgically removed.