朱丹溪曰∶或问双胎者何也？曰∶精气有余，歧而分之，血因分而摄之故也。若男女同孕者，刚日阳时，柔
日阴时，感则阴阳混杂，不属左，不属右，受气于两歧之间也。亦有三胎四胎五胎六胎者，犹是而已。
《人镜经》曰∶精气盛，则成二男。血气盛，则成二女。精血皆盛，则成一男一女。或精血散分，则成男胎。
或精血混杂，则成非男非女。男不可为父，女不可为母。皆非纯气，或感邪祟鬼怪之 气，则成异类矣。
朱丹溪曰∶或问有男不可为父，女不可为母，与男子之兼形者，若何分之？曰∶男不可为父，得阳道之亏
者也。女不可为母，得阴道之塞者也。兼形者，由阴为驳气所乘，为状不一。有女兼男形者，又有下为女体，
上具男之全角者，此又驳之甚也。或曰∶驳气所乘，独见于阴，而所成之形，又若是不同耶？曰∶阴体虚，驳
气易乘。驳气所乘，阴阳相混，无所为主，不可属左，不可属右，受气于两歧之间，随所得驳气之轻重而成形，
故所兼之形有不同也。
慎斋按∶以上三条，序受孕有双胎之异，有不成男女之形，此皆阴阳变常，驳气所感，理之所不可稽者也。
巢元方曰∶妊娠一月，名胚胎，足厥阴脉养之。二月名始膏，足少阳脉养之。三月名始胎，手心主脉养之。
当此时，血不流行，形象始化。四月始受水精，以成血脉，手少阳脉养之。五月始受火精，以成气，足太阴脉养
之。六月始受金精，以成筋，足阳明脉养之。七月始受木精，以成骨，手太阴脉养之。八月始受土精，以成肤革，
手阳明脉养之。九月始受石精，以成毛发，足少阴脉养之。十月，五脏六腑、关节、人神皆备。其大略也。
朱丹溪说：有人问双胞胎是怎么回事？回答说：这是因为父母的精气有余，分裂成两份，血液随之分流来滋养它们。如果是男女同孕的情况，在阳日阳时受孕则为男，阴日阴时受孕则为女；若感于阴阳混杂之时，则不属于左侧也不属于右侧，而是在两歧之间受气形成的。也有三胎、四胎、五胎、六胎的，道理也是这样的。
《人镜经》说：精气旺盛，则怀双男胎。血气旺盛，则怀双女胎。精血都旺盛，则怀一男一女。如果精血分散分布，则形成男胎（此处指单胎或偏阳之胎，原文语境稍显模糊，依文意直译）。或者精血混杂，则形成非男非女的胎儿。这种非男非女者，男子不能做父亲，女子不能做母亲。因为他们都不是纯正的阳气所化，若是感受了邪祟鬼怪之气，就会形成异类。
朱丹溪说：有人问“男子不能做父亲，女子不能做母亲”以及男子具有双重性征（兼形）的情况，该如何区分呢？回答说：男子不能做父亲，是因为得到了亏损的阳道。女子不能做母亲，是因为阴道闭塞不通。具有双重性征的人，是由于阴体被驳杂之气所侵扰，形态各不相同。有的女性兼具男性外形，有的下部是女性身体，上部具备男性的完整特征，这又是驳杂之气中非常严重的情况。有人问：驳杂之气所侵扰，难道只见于阴部，而形成的形态又如此不同吗？回答说：阴体虚弱，驳杂之气容易乘虚而入。一旦受到驳杂之气侵扰，阴阳相互混杂，没有主导的一方，既不属于左也不属于右，在两歧之间受气，根据所得驳杂之气的轻重程度而形成不同的形态，所以兼有的形态会有所不同。
慎斋按语：以上三条，叙述了受孕有双胞胎的差异，也有不成男女正常形态的情况，这些都是阴阳变化失常，受到驳杂之气感应所致，是常理中无法考证的。
巢元方说：妊娠第一个月，称为胚胎，由足厥阴肝经滋养。第二个月称为始膏，由足少阳胆经滋养。第三个月称为始胎，由手厥阴心包经滋养。在这个时候，血液不流行运行，形象开始转化。第四个月开始接受水之精华，以形成血脉，由手少阳三焦经滋养。第五个月开始接受火之精华，以形成气，由足太阴脾经滋养。第六个月开始接受金之精华，以形成筋，由足阳明胃经滋养。第七个月开始接受木之精华，以形成骨，由手太阴肺经滋养。第八个月开始接受土之精华，以形成皮肤和毛发（肤革），由手阳明大肠经滋养。第九个月开始接受石之精华，以形成毛发，由足少阴肾经滋养。第十个月，五脏六腑、关节、人神都具备了。这是大概的情况。
Zhu Danxi said: Someone asked, "What is the cause of twins?" The answer was: It is because the Jing-Qi (vital essence and qi) is in surplus, splitting into two parts, and the Blood divides to nourish them. If a male and female fetus are conceived together, conception on a Yang day or Yang hour results in a male, while conception on a Yin day or Yin hour results in a female; if conception occurs during a time of mixed Yin and Yang, it does not belong to the left nor the right, but receives Qi between the two divisions. There are also triplets, quadruplets, quintuplets, and sextuplets, which follow the same principle.
The *Renjing Jing* (Mirror of Humanity) states: When Jing-Qi is abundant, twin males are formed. When Xue-Qi (Blood and Qi) is abundant, twin females are formed. When both Jing and Xue are abundant, one male and one female are formed. If Jing and Xue are scattered and divided, a male fetus may result. Or if Jing and Xue are mixed, a fetus that is neither male nor female may result. Such individuals cannot serve as fathers or mothers respectively. They do not possess pure Qi; if they are influenced by evil spirits or strange energies, they become anomalous beings.
Zhu Danxi said: Someone asked how to distinguish between "males who cannot be fathers," "females who cannot be mothers," and males with dual sexual characteristics (Jianxing). The answer was: Males who cannot be fathers suffer from a deficiency of the Yang Dao (male principle). Females who cannot be mothers suffer from an obstruction of the Yin Dao (female principle). Those with dual sexual characteristics are caused by the invasion of mixed/impure Qi (Boqi) into the Yin body, resulting in various forms. Some females have male physical features, while others have a female lower body but a complete male upper body; this represents a severe case of impurity. Someone asked: Does the invasion of mixed Qi occur only in the Yin region, yet produce such different forms? The answer was: The Yin body is deficient, making it susceptible to mixed Qi. Once invaded by mixed Qi, Yin and Yang become mixed without a dominant aspect, belonging neither to the left nor the right, receiving Qi between the two divisions. The resulting form depends on the severity of the mixed Qi received, hence the variations in physical characteristics.
Shen Zhai commented: The above three passages describe anomalies in conception, such as twins or fetuses that do not develop into normal male or female forms. These are all due to abnormalities in Yin and Yang and the influence of mixed Qi, matters that cannot be verified by ordinary reasoning.
Chao Yuanfang said: In the first month of pregnancy, it is called Peitai (embryo), nourished by the Foot-Jueyin Liver Meridian. In the second month, it is called Shigao (beginning of fat/grease), nourished by the Foot-Shaoyang Gallbladder Meridian. In the third month, it is called Shitai (beginning of fetus), nourished by the Hand-Jueyin Pericardium Meridian. At this time, Blood does not circulate freely, and the image begins to transform. In the fourth month, it begins to receive the Essence of Water to form Blood vessels, nourished by the Hand-Shaoyang Sanjiao Meridian. In the fifth month, it begins to receive the Essence of Fire to form Qi, nourished by the Foot-Taiyin Spleen Meridian. In the sixth month, it begins to receive the Essence of Metal to form sinews, nourished by the Foot-Yangming Stomach Meridian. In the seventh month, it begins to receive the Essence of Wood to form bones, nourished by the Hand-Taiyin Lung Meridian. In the eighth month, it begins to receive the Essence of Earth to form skin and flesh, nourished by the Hand-Yangming Large Intestine Meridian. In the ninth month, it begins to receive the Essence of Stone to form hair, nourished by the Foot-Shaoyin Kidney Meridian. In the tenth month, the Five Zang Organs, Six Fu Organs, joints, and Shen (Spirit/Human Spirit) are all complete. This is the general outline.