Looking at tongue during prenatal appointment?

I bombarded my doctor today during my prenatal visit with a million questions and didn't want to ask her yet another one when it came to this part. Why, during prenatal visits does the doctor look at the underside of your tongue? She did the same thing with my last pregnancy- but it never struck me as weird until today. I tried to find out online and can't seem to find any reference to this. Anyone know?