Why do doctors test pregnant women urine for sugar and what will happen if high sugar in urine is found in the urine?

They are checking the woman for Gestational Diabetes, my friend.

If there's found to be a high concentration of glucose ... the type of sugar that's being tested for ... a blood test will be taken to confirm the result.

The reason this is done because when the pregnant woman has Gestational Diabetes it is extremely dangerous for both the mother and the unborn child ... moreso for the child.

Dependent on the level of glucose that's found to be present, the woman MAY be given advice on how to lower her 'sugar' OR MAY be put on insulin injections until after the birth of the child.

Having Gestational Diabetes increases the risk of both the mother and the child going on to develop Type 2 diabetes at some stage of their life.

It's called gestational diabetes. They'll be put on insulin until after the birth.

Urine sugar concentration is a marker for diabetes (i.e. high sugar content would mean a higher likelihood of diabetes).  I wonder if that's what they are checking for?  Could be other things too but that's probably the most likely.

they want to make sure their sugars dont get too high when theyre pregnant so it dont affect their babies

They're checking for possible diabetes II and gestational diabetes