According to researchers, there is a huge risk of a baby having birth defects if the mother has diabetes during or before pregnancies.
Over 400,000 pregnancies from north-east England were analyzed by experts for a study.
There was an increased risk of birth problems like congenital heart disease and spina bifida.
Sugar levels should be under check even before a woman plans to conceive, national guidelines recommend.
The amount of sugar can be difficult to control in both Type 1 diabetes, which tends to appear in childhood, and Type 2 diabetes, largely as a result of diet.
Problems like a miscarriage and baby's weight being too much can happen if sugar levels are not under control.
The problem can worsen with rising levels of diabetes, particularly Type 2.
From 1996 to 2008, about 401,149 pregnancies were analysed by researchers from Newcastle University and it was seen that 1,677 women had diabetes.
In their report they stated that controlling sugar levels before conception could avoid many problems.
The lead researcher, Dr Ruth Bell from Newcastle University, said, "Many of these anomalies happen in the first four to six weeks."