Being an overweight mother is not the only issue; being underweight can cause serious issues, too. Being underweight is less common in Western nations.
Being underweight can disturb your menstrual cycle to such a degree that getting pregnant can become very difficult. If your BMI is below 20 and you want to become pregnant, take additional starchy foods such as pasta, potatoes, and bread, as and protein-rich foods like fish, whole milk, and cheese.
An underweight mother is associated with, amongst other things, an increased risk of miscarriage or cardiovascular problems, and diabetes in the child at a later age.
If you come to pregnancy underweight, there can be up to a 280% increased chance of your child being of low birth weight, and up to 30% extra chance of premature birth. Even if this is not your first pregnancy, being a healthy weight is vital. In an Irish study it was found that women who are underweight between pregnancies are more likely to give birth prematurely.
It is vital that you keep your weight at a healthy level if you want a healthy pregnancy with fewer risks, in cases of severe low weight, it is advisable to speak with your GP who may refer you to a dietician.
From angelababy In Q&A