The handbook, 18.6.1 and 18.6.2, clearly instructs that a baby blessing is a Melchizedek priesthood ordinance.
“Melchizedek Priesthood holders gather in a circle to name and bless a child.” It would be inappropriate for someone that does not have the Melchizedek priesthood to participate in the circle by having a hand under the child or on the shoulder of a brother.
Holding a microphone is not a priesthood function. As mentioned - some wards have someone that holds the Aaronic priesthood hold the microphone. So, anyone could hold the microphone, to include a sister.
Giving the option for a mother to hold the microphone gives her an opportunity to participate with her husband, in their very first ordinance for their child. It is in line with recent changes to who can be a witness at a baptism.
We offer this option for sisters in our ward. Another benefit is that in larger wards, blessings can be very difficult to hear, this way the mother is right there for the blessing. And if they are husband and wife - they can present the child together after the blessing is done.
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