Yes that is totally normal. Everyone does it a different times. I think I was about 7 months when I started gaining a lot of weight.
I would not worry too much - I am sure they are monitoring the baby and checking for possible things like gestational diabetes (has she had her test for diabewtes yet?). I gained 9 pounds in my 4th month and then continued on with a normal weight gain (about one pound a week) for the rest of the pregnancy. Also, as losingmymind says, water retention is normal. Just make sure she is eating a healthy diet as too much junk food or sugar can really add weight during pregnancy.
retaining water is normal during pregnancy