From Proto-Indo-European *gʷrebʰ-. Cognates include Old Church Slavonic жрѣбѧ (žrěbę) and Irish bromach.
βρέφος • (brephos) (genitive βρέφεος or βρέφους); n, third declension;
- babe in the womb, foetus/fetus
This inflection pattern originally had a sigma (i.e. βρέφεσος), which was dropped early on in the history of Ancient Greek.
The irregular accentuation of this paradigm is explained by the fact that it is the contracted version of the above paradigm.