Gestational diabetes is a condition where the woman's body cannot produce enough insulin to cope with the raised blood sugar levels of pregnancy.
In almost all cases, it passes quite soon after the baby is born. (However, it can mean the woman develops diabetes later in life.)
Because all ante-natal check-ups include urine sample tests for 'sugar', it can be quite easily detected even if the woman has no history of diabetes.

What are the symptoms?
The presence of sugar in the urine, whilst occuring perhaps once during about half of all regular pregnancies, is deemed to be of concern if it happens at more than one sample test. (At this point blood tests or a glucose tolerance test can help flag up gestational diabetes.)
Other symptoms include the need to urinate more often, being exceptionally thirsty, and feeling very tired.
As the other symptoms can be part of any normal pregnancy, diagnosis would only be made after urine and blood tests.

What does it mean for my baby?
Gestational diabetes shouldn't be a serious cause for alarm once it has been raised as an issue, but it can affect the size of your baby. (Either born too small or very large, causing birth problems.) Or your baby might be more prematurely.

What can be done about it?
As gestational diabetes is quite common, it is a known danger and not particularly harmful to you or your baby, so long as you follow the care instructions of your GP or consultant, as well as exercising, and following a diet which is less likely to raise your sugar levels.
If this is not enough, you might be given insulin injections.
You will be monitored to ensure you do not develop pre-eclampsia, although this is also routinely checked for in all pregnancies anyway.
Provided the care team at your birth are aware of the condition there should be no great worries.