By 2100, that number is forecasted to jump to 97 percent of countries.
There are concerns there are not enough babies born to replace the existing population.
The National Party leader jokingly encouraged Kiwis to "have more babies" at an infrastructure conference.
Doctors describe the birth of the twins as 50 million-to-one.
The woman had intended to bring them to her mother's grave.
New Zealand's total fertility rate has reached an all-time low.
Genea Oxford Fertility is one of only two clinics of its kind in the South Island.
