Use the entire name on your birth certificate.

At or soon after birth, the baby's name is recorded somewhere. Most likely a birth certificate or other governmental birth record. Sometimes it may only be a notation by a family member in a keepsake book or other document or perhaps a church document.

"Name at birth" means the name as it was first recorded. Whatever was recorded at the time, even if the recording had misspellings or other typographical errors.

If the record includes the family name and/or middle names, then those are part of the name at birth.

The name as first recorded at birth has a lifelong effect on the person, even if the person later goes by a different name.