Never. If you are born that way, you cannot change it, no matter what you do to your physical self.
Now, there are plenty of womyn that were born with a physical birth defect and had it surgically corrected. Some of those defects involved their face, or their breasts, or their head, or their womb, or their genitalia. And none of those women was ever transsexual -- they were born a woman, and continued to be a woman before during and after their operation.
Only you know how you were born. If you say as a transsexual, I say fine by me. But if you say, I was born transsexual, but now I am a woman, I am not buying it, and neither will most other people.
We all define ourselves.