A study, published in Nature Medicine, found that high glucose levels make embryonic cells kill themselves even before implantion into the womb.

This loss of cells could help explain the higher rates of miscarriage and malformed babies among diabetic women.

Diabetic women have up to eight times as many malformed babies as other women.

Even if their blood sugar levels are under control by the time a baby's organs form, malformation rates are still two to three times higher than normal.

Study author Dr Kelle Moley said: "A lot of diabetic women figure they'll go to the doctor once they get pregnant.

"But by that time, the damage may be done. So it's very important for them to tell their doctor they want to get pregnant so they can be monitored very closely from that point on."

Dr Moley, a reproductive endocrinologist at Washington University School of Medicine, St Louis, said significant cell death would cause a miscarriage, and malformations would result from the death of just a few cells.

Cell suicide

Blood sugar levels have a big impact

The US National Institutes of Health does not permit human embryo research, so Dr Moley tested her idea on mice, whose development has much in common with that of humans.

Using blastocysts - the cellular stage before implantation in the womb - Dr Moley looked for signs of cell suicide, a process called apoptosis.

She measured levels of a protein called Bax, which triggers apoptosis.

Bax levels in blastocysts from diabetic mice or blastocysts cultured in high concentrations of glucose were five to 10 times higher than in those from non-diabetic mice.

But when diabetic mice received insulin before they became pregnant, their blastocysts had normal Bax levels.

Dr Moley also found much more evidence of DNA fragmentation in glucose-exposed blastocysts - a tell-tale sign of cell death.

Dr Moley's findings may also be pertinent to nondiabetic women.

"Even if we are not insulin-dependent diabetics, many of us have blood glucose fluctuations," she said.

"And pregnancy itself causes a lot of carbohydrate changes very early on.

"Perhaps even subtle metabolic alterations during this early critical time in development have serious effects on pregnancy outcomes. So if you're thinking of getting pregnant, you might want to pass up the soda and candy bars."

A spokesman for the British Diabetic Association said most diabetic women were perfectly able to have a healthy child.

He said: "We do recommend that women considering pregnancy should consult with their diabetic care teams three months prior to when they want to conceive so that there is an opportunity to ensure that blood glucose levels at kept as near to normal as possible."