noun
birth name
Refers to the name given at birth, especially when it differs from a later name (e.g., after adoption, marriage, or a legal name change).
彼女の出生名は田中花子ですが、結婚して山田花子になりました。
Her birth name was Hanako Tanaka, but after marriage she became Hanako Yamada.
旧姓 refers to a former surname, typically before marriage, and does not necessarily imply the name at birth.
Compound of 出生 (birth) and 名 (name).