"We have lost and grieved, and now have to do it all again."
It is understood the foetus was in the freezer, and had been for some time, while the family found a suitable place to bury it.
The woman's father, Dave Evington, told Fairfax his daughter could not carry the baby to full term because of medical complications which would endanger her life and give the baby a higher chance of being stillborn.
The theft had affected his daughter greatly, he said.
"My girl is a real strong person, but I can see it and hear it in her voice that it's not sitting well."
Hamilton police confirmed an amniotic sac had been reported as stolen alongside meat.
Senior Sergeant Gill Meadows urged the thieves to return the foetus.