In fact, this is the reason. Although Cao Guiren is often in Huafei Palace, in fact, Cao Guiren does not live in Yikun Palace. Of course, Hua Fei's Huan Yi Xiang is not lit every day, so Cao Guiren doesn't always smell Huan Yi Xiang. Besides, as we all know, the musk content of Huanyixiang is very small and it has been used for many years. It seems normal that Cao Guiren is pregnant.
Then why can she get pregnant? Can also give birth safely. The reason why she was able to give birth to the princess safely was also blessed by Hua Fei. As we all know, even if she can get pregnant, the queen will do it if Hua Fei doesn't do it. Huafei asked her to give birth to this child, but also for herself. If he can talk about the prince, Huafei can of course choose to raise the child by herself. In Hua Fei's words, "Talking is better than nothing".
Empress Hua Fei will never let the harem concubine get pregnant. She has no children of her own. How can she keep their children alive? Cao Guiren can have children because of her low status and no family to rely on. There are only two factions in the harem, the Empress and the Empress Hua Fei. She can only rely on the big tree of Empress Hua Fei. She has always been loyal to Empress Hua Fei, thinking of Hua Fei with all her heart. Although she didn't appear in the play, I think that Cao Guiren must be very thoughtful and a Chinese princess. Loyalty, pros and cons, even if she gave birth to a child, it is not a threat to Hua Fei at all. On the contrary, having a baby may be helpful to Hua Fei and is a good chess piece. Empress Hua Fei is overbearing, but when it comes to scheming, several Hua Fei are no match for Cao Guiren, so she has a chance to give birth to Princess Wen Yi.
As for Cao Guiren, you can't say that you like him or you don't like him. He is a poor man who tries his best to protect himself and his children in a deep palace. Therefore, it is not unreasonable to say that "once you enter the palace, it is as deep as the sea".
Of course, people like Cao Guiren have no identity, no background and no family. Even if they have children, they are not a threat to anyone, so it is not important to have children.