
I've read it to somewhere that Navi and Sovioushit is not totally 100% infertile maybe a bit of it but not totally. So when they two match up it's really hard to get pregnant but again when Heinly and Navi did it they use Magical Jade bed something which boost their chances of being pregnant.
It was also not said that Navier and Sivoush cannot have a child it's just maybe they didn't try hard enough to get one. Knowing well that they don't spend enough time with each other I think it's a more cleared and understandable reason why Soviosh didn't get Navier pregnant
added, I read this somewhere int he spoiler sites so I am not totally 100% sure if it's correct or not :D feel free to add up if you are sure
Okay I only have one question is that really his child or not?