代理母

Etymology 1
Compound of.

This construction treats as one word, and  as a separate word, hence the kun'yomi haha as used for  on its own.

Noun

 * 1) a woman who is impregnated and gives birth for another woman: a surrogate mother

Etymology 2
Compound of.

This construction treats 代理母 as one integral word, hence the on'yomi bo as used for in compounds.

Noun

 * 1) a woman who is impregnated and gives birth for another woman: a surrogate mother