Celestia found Spike's egg on a cushion pad floating in a river, until it came to rest near the Ancient Castle. She took the egg back to Canterlot, and watched over it. Until she was informed that a Unicorn was acceptted to her school for gifted unicorns, she sent the egg to the school, so the unicorn can hatch it.
After the Unicorn, Twilight Sparkle hatched the egg, Celestia made Twilight her student, took the baby Dragon, Spike in and raised him as her son.
But where is his natural, dragoness mother, who laid the egg in the first place? So doesn't he have three mommies? And how did Celestia get his egg? Was it abandoned? Did she steal it? Was it given to her? There's so many questions!