Over 600 pregnant women and new mothers are summoned back to hospitals after it was found that blood tests for diseases such as HIV were not carried out.
According to he National Blood Service (NBS) 627 women in the north of England were contacted about their tests for HIV, hepatitis B and syphilis.
The NBS said that the confusion arose because of the introduction of a new form two years ago used by midwives who tested women between December 1 last year and July 31 this year.
A spokeswoman for NBS said, "A number of tests were not carried out because the accompanying form which is completed did not indicate that the patient had given informed consent for the test."
Normally the NBS screens blood taken during pregnancy for diseases like HIV, however some primary care trusts use alternative providers for the service.
The spokeswoman said: "The NBS has searched records of all ante-natal testing referrals from 1st December 2005. So far investigations have uncovered 627 women who are affected by this, 506 whose tests were sent to Sheffield and 121 whose tests were sent to Leeds."
She reported that all NHS trusts involved had been contacted and had now written to the affected women. "Although the chances of them testing positive are low, it is important that they are offered the opportunity to have repeat tests."
The spokeswoman added that the NBS has now amended its procedures so that the tests are carried out where possible even if the form appears incomplete.