Persian (Farsi) to English Art/Literary Linguistics Dictionary Entry

Greetings,

In the Oxford Elementary Learner’s English-Persian dictionary, with regard to the English term ‘first name’, it says the following:
نامی است که والدین در بدو تولد می‌گذارند

Since, taking the words individually, در بدو تولد would seem to mean ‘at the beginning of birth’, which somehow seems odd, I'm wondering if this might be an idiom.

Best wishes, and many thanks,

Simon