In the annotations in Penguin's Ulysses: Annotated Student Edition, Kiberd says that "the six month of pregnancy" beings in the paragraph "Our worthy acquaintance, Mr Malachi Mulligan, now appeared..." (Chapter 14, page 525, line 5, in the edition in question).
He offers no explanation as to why he thinks this section begins the sixth month of pregnancy. Gifford, who mentions many other months in his annotations, says nothing about the sixth, or about this section in particular.
Why does Kiberd say that "This section begins the sixth month of pregnancy"?
This seems completely random on his part.