How can a couple repent for anal intercourse with the wife, is an expiation required for their act, can pregnancy result from this act, and what is the ruling on such a fetus?

Repentance for what occurred involves ceasing the act, regretting it, and resolving not to return to it, along with increasing sadaqah (charity) and good deeds, because "indeed, good deeds do away with evil deeds." There is no specific expiation for this sin. Pregnancy cannot occur through anal intercourse because it is not a pathway for a child.