I have now read two books from the 1800s in which single young men flirt with married pregnant women. (The first was Anna Karenina, in which Veslovsky flirts with Kitty Levin. The second was Middlemarch, in which Captain Lydgate flirts with Rosamond Lydgate.) The first time I encountered this, I just thought it was strange. When I saw it a second time, I started to wonder if there was a cultural value I was missing. (Without offering a personal reflection, I'll observe that the tone of this Wikipedia article suggests that attraction to pregnant women is a fringe thing in the present-day English-speaking world.) But historically, were pregnant women widely regarded as particularly attractive—to the point where it became a literary trope?
Pregnancy was a far bigger deal in the 19th century, when a woman's primary function was to produce children. Going back to Biblical times, the greatest fear for a woman was to be "barren." This was, of course, before the modern "DINK" phenomenon (dual income, no kids). A woman who was pregnant was then a "proven performer."
In her book, "Undercover Sex Signals," dating coach Leil Lowndes opines that we are in evolutionary terms only one step away from cave men. The logical followup conclusion is that good "caveman" traits, brute strength for men, fecundity for women, are highly attractive traits, even in relatively modern times, especially "early" modern times.