"I recommended a surgical exploration and the removal of that tissue," Starship paediatrician Professor Paul Hofman, who cared for the girl and is an author of the journal article, told the Herald.
"Thankfully her mother had the sense to ignore what I said."
The parents declined the "gonadectomy" operation and asked for no further follow-up until it was time for treatment to induce puberty, needed because of the supposed absence of gonads.
At 9 the girl was brought back to hospital after her breasts started developing. A gonad-related tumour producing oestrogen was suspected but tests ruled this out and natural puberty was confirmed.
An MRI scan identified normal female genital organs, including, unexpectedly, ovaries. Other tests confirmed she still had the male chromosome in her blood.
"She is a chimera, where you are made up of two separate individuals," Professor Hofman explained.
Twins in the uterus sometimes shared some blood. In the girl's case her brother's blood became "engrafted".
"The blood, which had some stem cells in it, which makes all the blood components, got into her bone marrow and, if you like, took over from her own bone marrow.
"Her bone marrow became [the same as] her brother's, just like a bone marrow transplant in an older person with leukaemia, their blood will be very different from the rest of their tissue."
Babies whose sex-chromosome in their blood was the opposite of their physical make-up had been rarely reported but it was probably more common than thought.
