Uh the "Birthday" Celebration is a count of seconds minutes weeks days months years since your Birth. Your Birth is the zero point of every second afterward. Traditionally expecting parents celebrate the birth before hand with a Baby Shower party that more specifically celebrates the female becoming a mother rather than the age of a new baby human. Fathers day and Mothers day are the continued celebration of being a parent.
You have a day of being born, this is your birth/birthing day. When you were born is therefore your first birthday. What we refer to as a first birthday, is really the second one.
Actually it’s not cause ur not alive yet until u breath on ur own and out of the womb so no it’s not really and the “first birthday“ is actually when u was not even developed yet so ur technically not even alive so no it’s ur first birthday that’s why u r one years old because it’s ur first birthday