Incredible piece. CPDL lists the text as
Præter rerum seriem
parit deum hominem
Nec vir tangit virginem
nec prolis originem
Virtus sancti spiritus
opus illud cœlitus
Initus et exitus
partus tui penitus
quæ disponit omnia
transfer in mysteria.
This is no normal scheme of things:
God and man is born of a virgin mother.
She has known no man;
the child's origin is unknown to the father.
By the Holy Spirit's power
this heavenly work has been brought about.
The beginning and end of your giving birth
who can really know?
By God's grace, which orders all things so smoothly,
your childbearing confronts us with a mystery.