Ab ovo is a reference to one of the twin eggs of Leda

Leda (mythology)

In Greek mythology, Leda was daughter of the Aetolian king Thestius, and wife of the king Tyndareus , of Sparta. Her myth gave rise to the popular motif in Renaissance and later art of Leda and the Swan...

 and Zeus

Zeus

In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...

, disguised as a swan, from which Helen was born. Had Leda

Leda

- Places :* Leda , tributary of the Ems in Germany* Leda, Western Australia, a suburb of Perth, Western Australia* Leda, Burkina Faso- Astronomy :* Leda , a moon of Jupiter* 38 Leda, an asteroid...

 not lain the egg, Helen would not have been born, so Paris

Paris (mythology)

Paris , the son of Priam, king of Troy, appears in a number of Greek legends. Probably the best-known was his elopement with Helen, queen of Sparta, this being one of the immediate causes of the Trojan War...

 could not have eloped with her, so there would have been no Trojan War

Trojan War

In Greek mythology, the Trojan War was waged against the city of Troy by the Achaeans after Paris of Troy took Helen from her husband Menelaus, the king of Sparta. The war is among the most important events in Greek mythology and was narrated in many works of Greek literature, including the Iliad...

 etc.

The English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 literary use of the phrase comes from Horace

Horace

Quintus Horatius Flaccus , known in the English-speaking world as Horace, was the leading Roman lyric poet during the time of Augustus.-Life:...

's Ars Poetica

Ars Poetica

Ars Poetica is a term meaning "The Art of Poetry" or "On the Nature of Poetry". Early examples of Ars Poetica by Aristotle and Horace have survived and have since spawned many other poems that bear the same name...

, where he describes his ideal epic poet as one who "does not begin the Trojan War from the double egg" (nec gemino bellum Troianum orditur ab ouo), the absolute beginning of events, the earliest possible chronological point, but snatches the listener into the middle of things (in medias res

In medias res

In medias res or medias in res is a Latin phrase denoting the literary and artistic narrative technique wherein the relation of a story begins either at the mid-point or at the conclusion, rather than at the beginning In medias res or medias in res (into the middle of things) is a Latin phrase...

).

This use is distinct from the longer phrase ab ovo usque ad mala (lit. "from the egg to the apples") which appears in Horace's Satire

Sermonum liber primus

The Satires are a collection of satirical poems written by the Roman poet Horace. Composed in dactylic hexameters, the Satires explore the secrets of human happiness and literary perfection...

1.3. It refers to the course of a Roman meal, which often began with eggs and ended with fruit. Thus ab ovo can also be used to mean a complete or entire thing.