birth:  Old English had a word gebyrd ‘birth’ which survived until the end of the 13th century as birde, but it was quite distinct from (though related to) modern English birth, which was borrowed from Old Norse byrth. This came from the same Germanic stem (*ber-, *bur-) as produced bear, bairn, and indeed Old English gebyrd. The suffix -th denotes a process, or the result of a process: hence birth is ‘(the result of) the process of bearing a child’. Along with bath and death it is one of the most ancient words formed with -th. => bairn, bear, berth
early 13c., from a Scandinavian source such as Old Norse *byrðr (replacing cognate Old English gebyrd "birth, descent, race; offspring; nature; fate"), from Proto-Germanic *gaburthis (cognates: Old Frisian berd, Old Saxon giburd, Dutch geboorte, Old High German giburt, German geburt, Gothic gabaurþs), from PIE *bhrto past participle of root *bher- (1) "to carry; to bear children" (cognates: Sanskrit bhrtih "a bringing, maintenance," Latin fors, genitive fortis "chance;" see bear (v.)).
Suffix -th is for "process" (as in bath, death). Meaning "parentage, lineage, extraction" (revived from Old English) is from mid-13c. Birth control is from 1914; birth rate from 1859. Birth certificate is from 1842.