佛祖統紀 2.13

2.13

序曰。託胎降生。皆所以示同於人法耳。至於出入右脇周行七步。俾大小兩機所見各殊者。斯示現降生之相也。

The Preface states: Conception and birth are both the means to manifest identity with human norms. As for issuing forth from the right flank and walking seven steps in all directions, causing what was seen by the two capacities of Great and Small to be distinct—this is the aspect of manifesting birth.