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Li Dongbi said: The *I Ching* (Book of Changes) states: "When male and female combine their essence, all things are transformed; the Way of Heaven forms the male, and the Way of Earth forms the female." Chu Cheng stated: If the blood arrives first and envelops the essence, a male is born; if the essence arrives first and envelops the blood, a female is born; if yin and yang (essence and blood) arrive simultaneously, it may result in a body that is neither male nor female; if essence and blood are scattered separately, it indicates the sign of twins or multiple fetuses. The Daoist Canon records: Conceiving on the 1st, 3rd, or 5th day after menstruation ends results in a male; conceiving on the 2nd, 4th, or 6th day results in a female. Li Dongyuan stated: When the Blood Sea (Chong Mai) has just cleared, conceiving on the 1st or 2nd day results in a male; on the 3rd or 4th day, a female. The *Shengji Zonglu* (General Records of Sage Relief) states: If Qi moves to the left and is nourished by Yang, a male is formed; if Qi moves to the right and is nourished by Yin, a female is formed. Zhu Danxi refuted Chu’s theory and supported Dongyuan’s, basing his argument on the *Shengji Zonglu*'s theory of left and right, attributing it to the ligaments of the left and right sides of the Uterus (Zigong). This can be considered thorough. However, I suggest that Chu’s view should not be dismissed, nor is Dongyuan’s entirely correct. Chu spoke from the perspective of the sequence of Qi and Blood arrival; the Daoist Canon spoke from the parity of days; Dongyuan spoke from the fullness or emptiness of menstrual blood; Shengji and Danxi spoke from the left-right position of the Uterus. Each adheres to one viewpoint; if they are integrated and understood together, the principle will naturally become clear. Can only single males or females be determined by days? Can twins or multiples also be determined by days? Historical records mention births of three or four children at once, with combinations of half male/half female, more males/fewer females, or fewer males/more females. Thus, is the saying "1st, 3rd, 5th days for males, 2nd, 4th, 6th days for females" really true? The views of Chu, Shengji, and Danxi regarding the relationship between essence/blood and the left/right of the Uterus are insightful, whereas the day-count theories of the Daoist Canon and Dongyuan are questionable. Wang Shuhe’s *Mai Jing* (Pulse Classic) distinguishes the sex of the fetus by the floating/sinking pulse on the left or right; Gao Yangsheng’s *Mai Jue* (Pulse Mantra) distinguishes twin/multiple fetal shapes by the crisscrossing/reverse pulse qualities: these are likely subjective speculations, not definitive arguments.

Wang Mengying added: Reading the *Weicaotang Biji* (Notes from the Micro Grass Hall), it says: A fetus is formed when two essences interact and merge. During intercourse, when emotions are harmonious, the essence arrives. If Yang essence arrives but Yin essence does not, or vice versa, conception cannot occur. If both arrive but at different times, the one arriving first loses its Qi and fails to grasp, so conception also fails. If neither is early nor late, and both essences arrive simultaneously, if Yang rushes forward first and Yin envelops it, Yang stays central as the master and forms a male; if Yin rushes forward first and Yang envelops it, Yin stays central as the master and forms a female. This is the wonder of natural biological transformation, beyond human control. Therefore, some conceive upon a single union, while others may unite thousands of times without success. This is what ordinary people know and can do, yet even sages may not know or be able to do—it refers to this phenomenon. Shen Xinpu, a descendant of Duanke, stated: The differentiation of fetal pulses is indeed knowledge physicians must possess. However, regarding whether one gets a male or female at the beginning of pregnancy, there is no fixed rule in the process of biological transformation. Although various discussions are ingenious, they have little practical clinical value and can be set aside.