False negatives could also be caused by testing too early in the pregnancy and testing after drinking too much and urine was diluted, among other factors.
"We provide full instructions to the health care professionals that administer these tests but understand sometimes these instructions are not adhered to.
"The notification to the MedSafe website is a positive step to encourage robust and thorough reporting to the regulators so if there is any product concern it can be fully investigated and the appropriate facts are available to MedSafe and Phoenix MedCare."
It was possible the product could be recalled if Medsafe deemed it necessary after further investigation.
The Auckland Women's Health Council had already demanded a recall, saying Medsafe's response of monitoring reports about the device and placing an early warning system note on its website was "pathetic".
"They should get it taken off the market," said council co-ordinator Lynda Williams.
Medsafe compliance manager Derek Fitzgerald said: "When investigating claims of problems with medical devices, [options] may include fault with the product, user error or the need for user education. Recall of the product could be considered after further investigation."