"Likely" is a bit of a stretch since odds of pregnancy after sex in a single day is only about 1-8% depending on stage of the woman's cycle and other factors. Still, it's AVN logic instead of real world, so actual probabilities don't really matter.
I had wondered about that myself because Merneith seemed EXTREMELY certain, in the original dialogue in 16 beta.
I had originally wondered if it was due to ovulatory cycle synchronization of women living together for extended durations in the palace, but it turns out sychronization is a completely discredited theory: sometimes there are just natural extended overlaps due to slightly different cycle durations.
A FAR more likely possibility, in fact VERY probable and ridiculously obvious when you think about it, is TALIA WOULD ALMOST CERTAINLY KNOW LIRAH'S EXACT CYCLE, as she works closely with her, every singe day. This would give the queen, via simply asking Talia, a near certainty of what the not-safe days were.