The first stage of the alien's lifecycle consists of an egg. The eggs are complex organisms in and of themselves. They are responsible for maintaining life support for the larva for an indefinite amount of time, and must recognize a potential host and distinguish it from valid members of the nest.
The egg can survive for an undetermined length of time in various environments. It appears translucent, is flammable, and unarmored. The egg itself is roughly one meter in height and two feet wide with four pedal-like lips at the top of it.
Eggs are laid by a Queen and they have root-like tendrils that extend from their base. Newly-laid eggs do not contain tendrils which suggest that they slowly develop over a period of time. Within an egg is a facehugger along with fluid and other materials to keep it alive. Once an egg has fully developed, its tendrils draw in nutrients from the surrounding environment to sustain the parasite after the limited supply of material inside the egg runs out. One problem that is unsolved is how the facehugger's waste is disposed of since there must ultimately be a by-product, but the egg may simply excrete it or turn it into more useable nutrients.
An egg's purpose is to detect a potential host, awaken the facehugger inside it, and allow the facehugger to leave upon locating the host. The egg detects when a host is proximal by sound and/or movement. As this happens, the egg comes to life, awakened by the chance to continue its lifecycle. The petals at the top of the egg open, preparing for the next stage as a face-hugger.
An egg's life span is unknown, but possible speculation is that it may be able to stay alive for centuries.