During the second trimester of pregnancy, a baby girl develops primordial follicles and oocytes - basically, these are the precursor to eggs that can develop, one of which will be ovulated during each monthly cycle after puberty.
When your mother was born, your egg cell was onboard. Mom had about a million such cells at birth, and one later became you! If you have any brothers or sisters by the same mother, their egg cells were likewise created at about the same time during your grandmother's pregnancy with your mother, even if you and your siblings are years apart in age. By the way, that's not a typo - it was when your grandmother was pregnant with your mother that your mother's egg cells were created; And, yes, as that implies, the egg that became you was within your grandmother!
Just like your mom, on the day you were born, you had your lifetime supply of eggs, some having the potential to become your future kids! And while those kids will likely not be conceived for at least several decades, their egg cells came into being with you.
