And it's unlikely (though not impossible) that Moonbox would choose to use a test from Tanya (or any side character) as a cliffhanger-style end for an update, as he'd also have to have some sort of ending for the players who chose not to get with that side character.Can't imagine how you come to that conclusion. Cat has as much chance of being pregnant as her mom ... maybe except for quantity per exposure (although there's definitely a new leader in THAT clubhouse ...)
There's a good chance that Aiko's pregnant, but unless there's people not living with Nana visiting the MCs bedroom when he's not home, the only options are Cat, Liz, Shizuka and Asami. Unless Tanya is testing for a third pregnancy? Not that I doubt she'll be proven to have that one when Anna 2 is born.
Someone from outside the home isn't impossible, but very unlikely.
And I'd only expect Tanya to have a third baby conceived before Anne's full sister is born if that sister is delivered before the end of the story (since we've been told Moonbox simply didn't want to have to create new models for a non-pregnant Tanya).