don't count your chickens before they're hatched

• don't count your chickens before they hatch, don't count your chickens before the eggs have hatched

First attested in English in Thomas Howell's 1570 New Sonnets and Pretty Pamphlets in the couplet "Counte not thy Chickens that vnhatched be, / Waye wordes as winde, till thou finde certaintee", possibly deriving from similar medieval and early modern Latin fables and maxims.

don't count your chickens before they're hatched

1. One should not depend upon a favorable (and typically overoptimistic) outcome to one's plans until it is certain to occur.

   Synonyms: don't get your hopes up, don't sell the skin till you have caught the bear

   • [1663, [Samuel Butler], “The Second Part of Hudibras”, in Hudibras. The First and Second Parts. […], London: […] John Martyn and Henry Herringman, […], published 1678, →OCLC; republished in A[lfred] R[ayney] Waller, editor, Hudibras: Written in the Time of the Late Wars, Cambridge: University Press, 1905, →OCLC, canto III, page 175:

     [...] Make Fools believe in their fore-seeing / Of things before they are in Being; / To swallow Gudgeons ere th' are catch'd, / And count their Chickens ere th' are hatch'd, [...]]

• it ain't over 'til the fat lady sings

• Gregory Y. Titelman, Random House Dictionary of Popular Proverbs and Sayings, 1996, →ISBN, p. 63.
• Jennifer Speake, ed., Oxford Dictionary of Proverbs, 6th ed., 2015, →ISBN, p. 60.