I am 28 weeks pregnant and wondering how my blood sugar, whether low or high, affects my baby?

Glucose effect. The fetal blood sugar will generally be about 2/3 to 3/4 of your level, which provides a needed nutrient for those fetal tissues that depend on glucose.Too low=baby must convert other stores to make up the deficiency & ends up small, too high & baby stores it in tissues and fat cells & can end up huge. There are also some birth defects related to excessive glucose exposure.
High can. High glucose is diabetes, and can have many bad effects for your child. Your doctor should be checking your glucose tolerance soon, as you are 28 weeks.