The Institute Of Medicine is widely recognized as setting guidelines for pregnancy. The last time they revised their estimates was in 1990.

Figure out the Body mass index (BMI). It doesn’t directly measure the fat, but research has shown BMI correlates with accurate measurements of body fat.

a.If BMI is less than 18.5= Underweight
b.BMI is 18.5 to 24.9= Normal or healthy
c.BMI is 25.0 to 29.9= Overweight
d.BMI is 30.0 or higher= Obese

If pregnant with ONE child:

a.Underweight: To gain 28-40 pounds (13-18 kgs)
b.Normal weight: To gain 25-35 pounds (11-16 kgs)
c.Overweight: To gain 15-25 pounds (7-11 kgs)
d.Obese: To gain 11-20 pounds (5-9 kgs)

If pregnant with TWINS:

a.Underweight: No guidelines due to insufficient data
b.Normal weight: To gain 37-54 pounds (17-24 kgs)
c.Overweight: To gain 31-50 pounds (14-23 kgs)
d.Obese: To gain 25-42 pounds (11-19 kgs)

During pregnancy many women gain substantially, the new guidelines now recommends an upper limit.

Reference: http://organizedwisdom.com/guidelines-for-weight-gain-during-pregnancy/4047034/nxi/med