#surely the emperor being infertile is a more foolproof method than targeting each consort who gets pregnant

[mydaylight]

The show didn't keep all the book details of Zhen Huan's poisoning of the Emperor in the last episodes, but there was actually quite a lot happening in the years following the aftermath of Yunli and Yuyin's deaths. Zhen Huan has Wei Lin use medicine to render Yongzheng infertile, so that he won't be able to produce any more children to threaten her and her sons. This also enables Zhen Huan - who is the de-facto empress at that point - to use a "divide and conquer" strategy in ruling the harem; she keeps throwing new concubines at Yongzheng to keep him occupied, but none of them are able to ever get pregnant. The result is that all the low-ranking concubines exhaust themselves in petty fights for favor while Zhen Huan's position, as the one with the most kids, is unattainable. At the same time Zhen Huan letting the Emperor have more consorts gets her a reputation for being virtuous and generous. This was actually one of Zhen Huan's most cruel moves towards the end, since she knows none of these girls have any future in the harem, and she's basically just waiting for Yongzheng to die anyways. I think this scene might indicate show!Zhen Huan is using the same strategy, although we don't see it be explained exactly.

(also I have to mention that one concubine comes up pregnant and Zhen Huan immediately knows the baby cannot be the Emperor's but her reaction is basically ¯\_(ツ)_/¯. She couldn't care less and even felt vindicated at hoodwinking him in this way)