Significant weight gain can cause a tattoo to stretch. In particular, women of childbearing years are often encouraged not to get tattoos in the midriff area because a pregnancy can cause enough dramatic stretching of the skin that will badly affect any tattoo.
When weight gain, whether due to pregnancy or not, causes stretch marks, any tattoos over the stretch marks will be ruined. The worse the stretch marks are, the worse the damage to the tattoo. Damage caused by stretch marks is irreversible. Significant muscle mass gain through heavy weightlifting may also affect a tattoo that has not been well-placed.