Il paio, le paia plurals with /a/ as a final come from the Latin neutral declination; I think there are no more than a dozen of such terms; other examples: il dito, le dita il braccio, le braccia Un euro, mille euro (does not change)

Uovo is like this: Lo uovo (maschile) le uova (femminile) If you ask about the gender, just check the article (if you know it), that will tell you the gender. The little problem here is that Italian lacks /neutral/ nouns so this Latin derivations, actually are similar to /maschile/ in the singular form and to /femminile/ in the plural form.