A surrogate mother is a
代孕妈妈 or, more formally, a
代孕母亲.
代 means "in place of", as in
代替 (to replace).
借腹怀胎 ("borrowing a belly to carry a child") is a more vivid way to say it. Mainland China has barred hospitals from performing surrogacy since 2001, so the word mostly appears in news about scandals and clinics abroad.
