Yep, exactly! When my pichu hatched, #172 Pichu became the last (highest) species entry in my pokedex, and #150-171 opened up as numbered "blanks" after #149 Dragonite (which I don't yet have, but have seen in gyms).

On YouTube, I've seen images of the pokedexes of people who've hatched the other babies; it looks like the same kind of expansion for them: up to the highest species number that they've caught or hatched or seen in a gym.