Answer/
Lines that look like red cracks.
Stretch marks form when the elasticity (ability to stretch) of the skin cannot keep up with the rapid growth of the belly, and the tissues deep inside the skin are torn. Simply put, the skin comprises three tissues: the epidermis, dermis, and subcutaneous tissue.
When the belly suddenly grows, the dermis and subcutaneous tissues are torn, and the color of the blood vessels and tissues deep in the skin show through, giving the appearance of red cracks.