I know it's so scary. I would wait until 6 months at least just to make sure all of her baby teeth have fallen out. If not your vet can extract them when she's being spayed so she'll only need the anesthesia once. I'm sure she'll do fine but us mommies worry.
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226