Symbolism, Imagery, Allegory
When Beatrice-Joanna flees to Mavis and Shonny's farm to wait out her pregnancy, they put her up in one of the "outhouses" (small sheds) on their property. Just in case the symbolism of a woman giving birth in a shed doesn't ring any bells, Anthony Burgess makes things obvious when Shonny remarks:
A greater mother than you gave birth in a— (2.9.24)
Mavis cuts him off, but we know that Shonny was going to say "manger," because it's clear that he's thinking of Mary, Mother of Christ.
So, not only is Mavis and Shonny's outhouse made symbolic through its association with the manger where Jesus was born, some of that symbolism rubs off on Beatrice-Joanna too. The novel implies that by giving birth in hardship and adversity, she's not only fulfilling her true nature and purpose as a woman, she's also following God's will in a wicked world.