Li Kaiyuan's book The Mystery of Qin Dynasty has a very detailed textual research on whose son Qin Shihuang was. You can have a look if you are interested.
Let's talk about the cause of this mystery first. All this still has to start from historical records. The biography of Qin Shihuang records that Qin Shihuang is the son of Zhuangxiang Wang He, and does not question Ying Zheng's life experience, but in another Historical Records? The Biography of Lv Buwei tells in detail how Zi Chu asked Lv Buwei for Evonne and how Lv Buwei got Zi Chu pregnant with a pregnant daughter. This story shows that Ying Zheng is actually the son of Lv Buwei.
But the truth of this story is doubtful. According to common sense, in Lv Buwei's view, Zi Chu's value mainly lies in his royal blood. Since Lv Buwei wants to ensure the value of Zi Chu as a rare commodity, it must ensure the purity of Zi Chu. How could he betray the royal blood?
From Mrs Huayang's point of view, she adopted Zi Chu on the basis of Zi Chu's royal blood. Qin's inheritance system is very strict, generally it must be inherited by the eldest son, and the identification of the eldest son must be born by the father and the main room. The purity of bloodline is related to the political future of Mrs. Huayang and the Zhou family.
Therefore, we must strictly supervise and screen any question about Zi Chu origin, leaving nothing that may be questioned. Lv Buwei will be careful on this issue and dare not neglect it. Qin is a country where the rule of law is supreme. On the fundamental issues concerning the country, people like Lv Buwei never dare to challenge the law, which is too dangerous for him.
And in terms of technology at that time, even if Evonne was pregnant, it was impossible to know whether it was a man or a woman. There is really no need for Lv Buwei to take such a big risk on this issue.
Another problem is that the birth time of Qin Shihuang recorded in Historical Records and Biography of Lv Buwei was after Evonne and Zi Chu lived together, and there were no children. So he married Ji. "Big period" is the date when a woman gives birth at full term. Although the exact length of this "big aunt" is still controversial, generally speaking, ten months pregnant is a woman's big aunt. According to this calculation, Evonne and Zi Chu lived together for ten months and gave birth to Ying Zheng. No problem, so Zi Chu acknowledged his eldest son and made Evonne his wife.
If Evonne was pregnant before she got married. But in ancient times, when there was no scientific testing method, it took at least two months to know whether you were pregnant or not. In other words, Zhao Ji will be pregnant for two months before she can dedicate it to Zi Chu. In that case, Ying Zheng won't be born ten months after her period, but she should be born prematurely after living in Evonne with Zichu for eight months. Will Zi Chu be stupid enough to be two months premature? Does he still think it's okay?
Another point is that the story of Xian is exactly the same as that of Li Yuan and Chun. In historical records. The Biography of Chun Shen Jun records that King Gao Lie of Chu had no children. After hearing the news, Li Yuan, a Zhao native, first betrothed his sister to Chun. After Chun became pregnant, Li Yuan persuaded Chun to betrothed his sister to King Gao Lie. Later, after Li Yuan's sister was appointed as Li Yuan, her son became Chu Youwang.
Therefore, although both Historical Records and Zi Zhi Tong Jian recorded the statement that Lv Buwei was the biological father of Qin Shihuang, there were too many loopholes in this statement. Combined with the textual research of many experts and scholars, Qin Shihuang should be the son of a foreign race who won.