It should be noted that every version of this is extremely heavily modernized. Nor is there much reason to think it is traditional, except possibly for the company it keeps. There is only one source, Cambridge, Caius College MS. 383, from the fourteenth or fifteenth century. It contains no other verifiable folk songs, though there were two others that I included in the Ballad Index as items that might be folky.

Richard Leighton Greene, editor, The Earliest English Carols, Oxford/Clarendon Press, 1935, p. xcv, while admitting he has no proof, thinks this one of two carols in Caius 383 that, "because of their homeliness, their directness of speech, and their theme of the betrayed girl, have a strong case for consideration as authentic folk-song." But other equally eminent scholars have disagreed.

The poem is written as prose in the manuscript. The text is old enough that it apparently still used þ and &yogh; instead of th and gh, and u/v and i/j/y are still one. I'm not going to try to transcribe it all, since I've had trouble with unicode on mudcat before. But I'll give you a couple of verses, as transcribed in Rossell Hope Robbins, Secular Lyrics of the XIVth and XVth Centuries, p. 24.

Wybbe ne rele ne spynne yc ne may,
ffor ioyȝe þat it ys holyday.

Al þus day ic han souȝt
spyndul ne werue ne wond y nouȝt
To myche blisse ic am brout
aȝen þis hyȝe holyday.

There are ten verses. The last is

sone my wombe began to swelle
as greth as a belle;
durst y nat my dame telle
Wat me betydde þis holyday.

That is one of the easiest verses to modernize, so I'll modernize it so you'll know I'm not kidding about that being the song. :-)

Soon my womb began to swell
As great as a bell
Dare I not my dame tell
What me betide this holiday.