Developing good eating and nutritional habits should start in the first trimester, despite the nausea and decrease in appetite you may have.
Just a few basics – if you are starting from an average weight (BMI of 25), you will only require on average an additional 300 calories a day (assuming you were consuming 2,500-3,000 calories a day prior to pregnancy).
The amount of weight you should gain depends on how much you weigh at the beginning of the pregnancy. For example, if you are an average weight you can expect to gain between 25-35 pounds.
Most experts recommend you maintain a well balanced diet with adequate protein (and/or iron supplementation) intake to prevent iron deficiency. This should include vitamin D and calcium containing foods.
Folic acid is also very important to prevent neural tube defects.
It is recommended you take 400 mcg daily (or 4 mg daily if you’ve had a child with a neural tube defect in the past).
Most prenatal vitamins contain an adequate amount of vitamins (including folic acid, etc.) and minerals to supplement your diet. Many on the market today also contain DHA – an omega-3 fatty acid that has been proven to help with fetal brain development (300 mg daily if taken separately).
Additional supplements – especially Vitamin A – may in fact be harmful.
So, when in doubt – ask your physician about what the current recommendations are.