Women with diabetes are not receiving enough support from the NHS before pregnancy, as services are "poor and uncoordinated", a report says.
Pregnant women with diabetes have a higher risk of complications
The Confidential Enquiry into Maternal and Child Health found many were not given advice about controlling their condition when trying to get pregnant.
The study of 521 women's case notes found under 50% of them were taking the necessary nutritional supplements.
The government said it was committed to reducing the risks during pregnancy.
Previous CEMACH reports have found poor standards in maternity services and revealed women with diabetes were five times more likely to have stillbirths.
The latest study, covering Wales, Northern Ireland and England, found that women did not appear to be receiving enough support from their GPs and the NHS despite the government laying out guidelines under a national service framework in 2001.
In particular, CEMACH said pre-conception care was "poor and uncoordinated".
Fewer than half of the women were recorded as having taken the extra folic acid required before getting pregnant.
A similar number did not receive counselling about controlling their condition and what diet to follow.
Researchers said there was a basic lack of knowledge about how to treat women with diabetes, but they also suggested tight finances in the health service could be playing a role.
Over one-third of hospitals did not have multi-disciplinary diabetes ante-natal teams as set out by the national standards.
Researchers said care for women with type 2 diabetes was worse than those with type 1.
They said access to services must improve and women with diabetes must be given better education about how to look after themselves.
Dr Jo Modder, CEMACH's clinical director, said: "This report provides clear evidence that women with diabetes are not being empowered to prepare adequately for pregnancy, and that health professionals are often not providing the appropriate information and care."
Douglas Smallwood, chief executive of Diabetes UK, said: "We need to work together to ensure that women with diabetes and people involved in their care are well equipped and are taking all necessary steps to minimise risks."
Dr Sue Roberts, the government's national clinical director for diabetes, said: "We are committed to ensuring that risks during pregnancy for women with diabetes are minimised.
"The recommendations in this report make a valuable contribution to the care of women with diabetes who are pregnant."