Actually, from Ewen's POV it would in some sense be good if Alison was pregnant, if he's going to die! Because then he would have an heir (well, at least if it was a boy). I'm sure Alison is brought up to see this as an important charge, as well, though if she's having second thoughts, this would be the moment to break it off. But she wouldn't be alone--she could return to Ardroy before giving birth and have the support of Aunt Margaret, and manage Ardroy until her son (if it was a son) came of age. If it was a daughter, we're in a different situation, of course, and that's a gamble! (Heh, this is the patriarchy in a very literal sense...)

This is almost the same situation that Ewen's parents were in after the '19...Ewen's father fled the country when Ewen was only three days old, leaving his wife and son, and no one knew if he would ever return, and in fact he doesn't.

But yeah, I do think Alison and Ewen do have differing fears and hopes in that moment. One fears having something and then losing it, and the other fears never having had it...