for all we know, the magic inherant to the sions (children of sovereign and a mortal) causes a pregnant female sovereign to become even more powerful while pregnant (or at least compensates for any difficulty due to pregnancy).
And a sovereign can teleport to safety when s/he is about to be killed, maybe he/she could probably whisk his wife/her husband away too. so really, no special changes need to be made.
It probably would make more sense if the Child was born in the same tile as the channeler, but ah well, its much more conventional for the wife to remain in the capital, and thus that is where the child will be born.
My female sovereigns would have something to say about that...
I am pretty sure he meant wives of male sovereigns... likewise, the husbands of female sovereigns should stay in the capital where they are safe instead of be taken to the battlefield (assuming the sovereign cannot use a single escape spell to teleport both of them away from combat). The sovereign, male or female, pregnant or not, can teleport to safety and is never in real danger; also is of near godlike power... no real reason a pregnant sov can't be on the front lines. but their mortal spouse should be kept safe; so I really don't think he meant it as "female sovereigns should stay in the kitchen" kind of comment.
give birth on the battlefield is definitely a "rule of cool" thing that should be accounted for (even if it gives no mechanical bonuses, just keep a record of it; aka, each person has a "born at" field, which can be a city, a location between cities, or the battle of X).
One of the problems I see is that sovs live ~800 years and near godlike in power, and their children, the sions, have significant powers as well. Having ONE mortal spouse and thats it, forever, seems silly on several fronts. First there is the whole notion that you never fall in love again in 800 years, second if you ignore the romance issue, there is the assumptions of a monogamous (vs polygamy) marriage (vs bachlor life with many sexual partners) on the part of the sov... also perfect loyalty to a mortal spouse extending even beyond their death hundreds of years ago, and finally, not utilizing the benfits of the powers of the sions.
having many sion children (who possess magical powers) would increase the power of your empire. It makes sense for both male and female sovs... and gives quite the advantage to a male sov (ghengis khan has sired over 1000 children; he wasn't the only male ruler to have done so). So as cool as it sounds, sions should NOT be born with ANY essence. They are mortals who might be slightly more likely (no in game effect) to sire a sovereign, but they are mortals. This means there is no tactical advantage to having more children (other then the usual "marry them off to someone")