Most snails are indeed born with a shell; this shell is known as a protoconch. The word protoconch literally means “first, earliest, or original shell” and is often used when referring to an embryotic or larval shell of some classes of mollusks. However starting off this shell is transparent and soft to begin with.
This requires snails to need a lot of calcium. This consumption of calcium then helps to harden their shells. In fact the very first thing a newly hatched snail often does is eat its own casing of its egg in order to absorb calcium and ensure good health, nutrition, and hardening of the shell. Some snails have even been known to become cannibals and eat the shells of their unhatched siblings.
Over the next three months or so the snail’s shell begins to thicken up and typically begins to grow into a spiral shape. However snails are also known to have different shaped shells and some come in rounded, flat, pointed or high spiraled shapes, it just depends on the species. When growing into a juvenile snail it will also eventually acquire the full adult coloration and an opening will be added to the shell. The part of the shell that the baby snail was born with typically ends up in the middle of the spiral. When some species of snails become too big for their shells they may abandon it and use one from another animal or a previous snail that has shed its shell.