"The difference is now - and this is something a lot of mums who have been through a loss feel - I don't have that kind of naive outlook on life any more," Mrs Topperwien said.
"I know that bad things can happen and I know bad things can happen to my children, so I was worried about the pregnancy."
Zayn's slightly early arrival on January 30 brought immense joy to the couple and their families, but there was still a sense of loss.
"It's obviously made us really happy and given us something positive, but at the same time Chace is always on our minds," Mrs Topperwien said.
"We were happy and sad," Mr Topperwien said. "We were happy that Chace had a brother but sad that Zayn didn't get to meet him."
Though he resembles his brother, Zayn was "growing into his own little person", Mrs Topperwien said.
They named him Zayn Chace, after his brother, who would have turned 5 next Monday.
Today is the third anniversary of Chace's cancer diagnosis, which made headlines because at his six- week immunisations a nurse mistakenly injected him with Gardasil, a vaccine to prevent cervical cancer.
By the time Chace was 2, he had developed a rare and aggressive strand of acute myeloid leukaemia, but doctors said they couldn't know whether it was related to the Gardasil.
The Topperwiens raised tens of thousands of dollars for treatment for their son, including for a drug trial in the United Kingdom.
On Tuesday, Zayn also had his six- week immunisations and Mrs Topperwien said they asked nurses to triple check the contents.
Meanwhile, the growing Dream Chaser Foundation is now helping 60 families.
This weekend, the couple will commemorate Chace's birthday with a "pressie drop" at the oncology ward of the Starship hospital.
Among the charity's other projects is raising awareness of bone marrow donation, especially among Maori and Pacific Island people after no match could be found for Chace.
• For more information about the charity, visit dreamchaser.co.nz
