Etymology

ON had masc. and fem.nouns in /ð/, represented respectively by OIcel burðr (in the sense ‘burden (on account of pregnancy), foetus, offspring, being; delivery, birth’), Far burður, Norw bur(d), Sw börd (masc.), and (rarer) OIcel byrð ‘birth, ancestry’, OSw byrþ, Dan byrd (fem.). Since WGmc has a range of corresponding fem. forms with stops, including OE gebyrd (and cp. OFris berde, OS giburd, OHG giburt), there is a strong case to be made for ON input behind ME burþe and similar forms with spellings implying a fricative. It is not, however, a sure test of loan as there is some evidence for Gmc cognates without Verner's Law in the suffix, thus (perhaps) Go ga-baúrþs and more clearly OFris berthe, and cp. further related derivations on the *ƀer- root, esp. OE byrðen ‘burden’ and beorþor ‘pregnancy, childbirth, offspring’. Thus a number of explanations are possible without invoking ON input: the ME form could originate in a native form without Verner's Law, it could be influenced by related words in OE /ð/, or its suffix could have been reformed by analogy with OE words in the abstract -þ(u) (e.g. lengþu ‘length’).  

PGmc Ancestor

*burþ-

Proposed ON Etymon (OIcel representative)

burðr  ‘burden (on account of pregnancy), foetus, offspring, being; delivery, birth’, byrð ‘birth, ancestry’
(ONP burðr (sb.), byrð (sb.))

Other Scandinavian Reflexes

Far burður, Icel burður, Norw bur(d), Dan byrd, Sw börd

OE Cognate

 gebyrd 'birth'; cp. byrðen ‘burden’ and beorþor ‘pregnancy, childbirth, offspring’

Phonological and morphological markers

[ON fricative /ð/ < PGmc */ð/] (may not be applicable)

Summary category

CC2a

Attestation

Forms with fricative -th- (etc.) are common and widespread in ME (cited by MED from the Trin.Hom. onwards). The clearly native -d- variant is found mainly in early ME (both W and W texts, inc. Orrm).

Occurrences in the Gersum Corpus

Gaw 922; Pe 1041; WA 256, 289, 439 etc.

Bibliography

MED birth(e (n.) , OED birth (n.1) , HTOED , Dance burþe, Bj. 162, byrð (sb.), de Vries burðr, byrð, Mag. burður, byrð, Seebold ber-a-, Bammesberger 141, Orel *ƀurðiz, Kroonen *burdi-, AEW gebyrd (1), DOE gebyrd