Glucose levels rise. High blood sugar before pregnancy means either pre-diabetes or type 1 or type 2 diabetes. If not already treated (such as for pre-diabetes) prior to conceiving, then the future pregnancy state's various placental hormones can cause the glucose levels to increase out of control. High glucose can cause problems for baby: excessive weight, hypoglycemia at birth, higher risk of future type 2 dm, etc.
Increase rate. Poor control of diabetes prior to conception is tied to increase incidence of miscarrige and fetal malformations. Diabetes is like going to the bank, if you have good control before(good reserves) you have a better chance of doing well when hard times come. Having good control just after conception will not erase the damage done.