figlio illegittimo significa in inglese

love child▼(child born as a result of a romantic liaison between unmarried parents)
noun
[UK: ˈlʌv tʃaɪld] [US: ˈlʌv ˈtʃaɪld]

natural child [natural children]▼(child born to unmarried parents)
noun
[UK: ˈnæt.ʃrəl tʃaɪld] [US: ˈnæ.tʃə.rəl ˈtʃaɪld]