The Gemoro learns from the phrase "miBen Chodesh VaMaalah" that pidyon is only for those OVER 30 days. I would have thought in translation of the words the opposite, that pidyon is for those "Ben Chodesh" and also for those above Ben Chodesh! (Unless the "VaMaalah" is a yitur, and the posuk could have said just "miBen Chodesh" ?)
[My question also apparently applies in the case of Arachin, see Mishna Arachin 18a and Rashi there]
Kol Tuv,
Mark Bergman, Manchester UK
I think you answered your own question in the parentheses. As an aside, not everyone understands that this is the explanation of our Gemara (see Shach and Bach Y.D. 305 who hold that the baby has to be more than a standard lunar month, meaning 29 days, 12 hours, and 793 Chalakim, unlike the Magen Avraham 339:7 who understands as you did).
Kol Tuv,
Yaakov Montrose