The right weight gain will help you stay away from several pregnancy complications.
If you gain less than the recommended weight range, you may deliver an extremely small baby because of insufficient nourishment. Babies born too small will have difficulty with breastfeeding, high risk of illnesses and will experience a delay in development (they cannot reach the milestones of their age).
If you gain more than the recommended weight range, you will deliver a big baby, and it again leads to complications like caesarean section and obesity. Also, you will remain obese even after pregnancy.
BMI of less than 18.5 is normally considered underweight, and more than 25 is overweight. More than 30 is obese.