Jamieson, Fausset and Brown on Isaiah 66:7

7. she--Zion.

Before ... travailed ... brought forth--The accession of numbers, and of prosperity to her, shall be sudden beyond all expectation and unattended with painful effort (Is 54:1, 4, 5). Contrast with this case of the future Jewish Church the travail-pains of the Christian Church in bringing forth "a man child" (Re 12:2, 5). A man child's birth is in the East a matter of special joy, while that of a female is not so; therefore, it here means the manly sons of the restored Jewish Church, the singular being used collectively for the plural: or the many sons being regarded as one under Messiah, who shall then be manifested as their one representative Head.