When Judson Laming has his 21st birthday party this October, he will have two "mothers" there to celebrate with him.

One he calls his mother, the other is his surrogate mother, who he knows as his aunty.

The South African-born Christchurch student - who hopes to become a physical education teacher - partially owes his existence to "Aunty" Natalie Brebner, who was a sister-in-law to his parents when she offered to be a surrogate.

Having been unsuccessful in conceiving a child themselves, Judson's parents Les and Jeanette Laming turned to Ms Brebner in the late 1980s while living in South Africa.

"We were just sitting at the grandparents' house one day - and [Mrs Laming] just says she wishes she could have children," Ms Brebner told the Weekend Herald.

"And I just made a passing comment 'oh, I'll have one for you one day, don't worry about it'. About two years later or whatever it was, she said: 'Remember that time that you said...

let's do it'. And it started from there."

The process started in 1989, with Ms Brebner's own ovum (egg) artificially inseminated with Mr Laming's sperm. In October 1991, Ms Brebner delivered twins early at 32 weeks - one of the babies, Chaz, died four hours after birth.

Judson was the surviving twin, and was adopted by the Lamings.

Judson said people were always fascinated by his life story - "about who did what - you do kind of have to draw a diagram to explain it".

It is a life story he is lucky he can still tell because he was studying with the New Zealand Institute of Sport, which was based in the Canterbury Television (CTV) building that collapsed in the February 2011 earthquake. On the day of the quake, he and other students had gone out of town on a camp.

"It's crazy. If it had happened 24 hours earlier..."

Ms Brebner, 46, who now also lives in Christchurch, said there was no issue for her handing Judson over to his new parents after giving birth to him.

"He was born for a purpose. He had a direction. We never harped on it. It was just something that was done. The words don't have to come out - you know that [the parents] are grateful."