This isn't really a plot hole just an observation

*spoilers*

If the trow were so dead set on murdering the mothers of their children why the heck were they marrying these women?
I had wondered if marriage was just part of the ritual but Danna wasn't married and she was impregnanted by one of the Trow.
Wouldn't it be like 100× easier to find some back alley surrogate they had no emotional attachment to?
Not to mention it would be significantly less suspicious than having to constantly pay people off and make excuses for why all your wives keep dying.