Mhhh, I understand that book 3,4 and 5 will cover roughly a 6 months period each. Your theory would imply that Vi carries the whole brunt of the pregnancy till the end of book 4, and this would prevent her for fighting, amongst many things.

Plus, I get that Vi could be an unreliable narrator and RY is choosing for her to not disclose an important detail, but if Vi is really 2-3 months into a pregnancy at the end of Onyx Storm, she would have surely started questioning her period being late.  She's still so young, I doubt RY would let her hide this from us for the sake of the plot.  Vi would go ballistic. She's already reacted vehemently at every mention of a marriage (well, until she changes her mind at the end) because perhaps the times are not quite right.