There is some developmental benefit to spaying later, like filling out and growing less tall (this is really only crucial to large/extra large breed dogs). But all that stuff is pretty insignificant, statistically speaking, for the average pet dog. So - definitely BEFORE first heat.
Don't worry; Sunny will be perfectly fine. Come to think of it, Ivy has "gone under" three times already in the ~1 year we've had herI'm a little desensitized to it all heh..
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