Zhao's fertility problem needs to be explored in segments. In his youth, he did give birth to five daughters and a son. However, during the Battle of Jingkang, five daughters were captured by Jin soldiers, three of whom died on the way, while the other two suffered a deep ordeal after being sent north.
Zhao's only son, Zhao Xuan, was born in 1127. Soon after, the Miao-Liu Mutiny broke out in the Southern Song Dynasty, resulting in the three-year-old Zhao Xun being elected emperor, but soon "abdicated" to his father Zhao Shu. However, the mutiny caused deep psychological trauma to Zhao, who fell ill soon after and died in July of the same year. Zhao Xun's death plunged Zhao into despair, and although he worked hard in the harem, he had no successors.
The official history does not record in detail why Zhao was unable to have any more children, but folklore has spread various theories. There is a theory that Zhao Shu was greatly frightened by Jin Bing's pursuit, which led to an unspeakable illness that made him infertile. Although this statement is not credible, it also reflects Zhao Shu's dilemma.
On the other hand, Zhao Zhi chose to adopt two clan children after Zhao Xun's death, Zhao Bojun and Zhao Bojiu, which is not common in history. Zhao was only 26 years old at the time, in good health, but he was anxious to pass on his adopted son. This may be because he himself has realized the problem of fertility, so he needs to make inheritance arrangements in advance.
Although Zhao actively sought medical treatment, and he even promoted a doctor, he was ultimately unable to become his biological father again. In 1162, Zhao decided to pass the crown prince to Zhao Bojun (赵昚), and abdicated himself as Emperor Taishang. Finally, in 1187, at the age of 81, Zhao Zhi died, and I don't know if he still had regrets before he died.