The majority of pregnant women with diabetes have blood sugar levels that are so high they could potentially damage their baby, according to a new report.
Too many women have high blood sugar in early pregnancy, with 85% of women with Type 1 diabetes and 64% with Type 2 having higher than recommended blood glucose levels, it said.
Furthermore, almost 12% of women with Type 1 diabetes and 8% with Type 2 diabetes in the study had blood glucose levels above the point at which women are advised to avoid becoming pregnant.
High blood sugar increases the risk of stillbirth, neonatal death and babies being born with congenital abnormalities.
The National Pregnancy in Diabetes audit, covering England Wales and the Isle of Man, also found that half of women with Type 1 and two thirds of women with Type 2 diabetes are not taking folic acid when they become pregnant.
Folic acid (also known as vitamin B9) is important for the development of a healthy foetus, significantly reducing the risk of neural tube defects such as spina bifida.
Women with diabetes need to take a higher dose of folic acid than other women, and this must be prescribed by a doctor or nurse.
Experts behind the study said diabetes teams, general practices and maternity services must now work together to increase awareness of the issues among women with diabetes.
A third of babies born to mothers with diabetes already need intensive care or specialist care.
The study of 2,537 women with diabetes who were pregnant in 2014 in England, Wales and the Isle of Man, found that one in 10 with Type 2 diabetes were taking medication that could harm their baby when they became pregnant.
But the report did stress that congenital abnormailities are relatively rare.
Lead clinician, Dr Nick Lewis-Barned, said: "There are three key elements of pregnancy preparation for women with diabetes to reduce the risk of adverse pregnancy outcomes - good control of blood glucose levels, taking folic acid supplements and a medication review.
"It's clear from the audit that many women need more information and more support in all of these areas."
Chris Askew, chief executive of Diabetes UK, said: "It is deeply worrying that so many women with diabetes do not have their condition under control during the early stages of pregnancy, as this is putting the health of the baby at risk.
"The clear message of this report is that many women with diabetes are not getting the advice and support they need when it comes to planning to become pregnant and the stark fact is that in too many cases this is leading to tragic consequences such as death or disability of the baby, with a third of babies born to mothers with diabetes needing intensive or specialist neonatal support."
He said doctors and nurses must review medications being taken by women with diabetes who want to become pregnant.
He added: "With good planning and the right care and support in place, women with diabetes can have healthy pregnancies and healthy babies."
In the UK, there are 3.9 million people who have diabetes and a further 590,000 people with Type 2 are undiagnosed.
The National Pregnancy in Diabetes audit is managed by the Health and Social Care Information Centre in partnership with Diabetes UK supported by Public Health England.