Going on a crash diet right before you start trying to conceive is just plain bad news. While your objective may be noble — getting to a healthy weight before you embark on a pregnancy — you risk depleting your body of important nutrients at the very time when those nutrients are needed to grow a healthy baby. In fact, if the Fad Diet Du Jour that you’ve been following is overly strict, you may even stop ovulating entirely — your body’s way of switching into self-preservation mode in the face of a perceived “famine.”
Note: If you’re struggling with an eating disorder such as anorexia nervosa or bulimia nervosa, or if you have battled with such an eating disorder in the past, you’ll want to be upfront with your doctor or midwife about the situation. Studies have shown that women with eating disorders face an increased risk of miscarriage, obstetrical complications, of giving birth to a low-birth weight baby, and of developing postpartum depression.
That’s not to say that these problems are unavoidable, of course: you may simply require some added psychological support (and possibly some nutritional counseling as well) during your pregnancy.
Source : The Unofficial Guide to Having a Baby. Second Edition . Ann Douglas and John R. Sussman, M.D. 2004