It is a defect wherein a woman is born without a uterus, cervix, and, in many cases, a vagina, and therefore cannot reproduce. The woman is also likely to be missing one kidney. No exact figures are known, as the defect often goes unnoticed by anyone outside of the immediate family of the woman.
Figures range from one in ten thousand to one in a million.
You just have it. There is no evidence that it is inherited, nor any evidence linking to anything that the mother did while the MRKH baby was in utero. MRKH is usually diagnosed when the girl reaches sixteen or thereabouts and never gets her period. Sensitive material dealing with women's birth defects is dealt with on this site. If you are offended by this, please go back to whence you came.