She's been dropping hints about wanting another baby for a long time now. For years she insisted that SB would be one and done, just like her recipes, but that has changed...
For anyone who wasn't around then: in November, Jack posted on Twitter asking for girls' names beginning with C. She was actually talking about her new kitten (the one she lied about and now claims she was never going to keep). This led to a clickmill site churning out an "Is Jack Monroe pregnant?" article, which concluded that she wasn't. Nevertheless, Jack was HOWLING AND CLAWING, claiming journalists were emailing her for comment. She tweeted this:
There have been several more hints about babies and ovaries over the last year.
Incidentally, that rant presumably pre-dates Harold (speculation, but based on lights going on and off and breadcrumbing about special meals etc).