Eating too much can be a problem during pregnancy because women figure we're going to blow up anyway, so why not indulge? My OB/Gyn was not a "weight Nazi" which in hindsight, I wish she were.
But the REAL problem (assuming the mother is a non-smoker) is coffee. I believe coffee (which I was NOT warned about) caused my daughter to be born with an arrhythmia. I didn't cut down at all on caffeine (this was 14 years go). She was first discovered to have the arrhythmia with 2 weeks remaining in utero. I had to cut out coffee completely (which caused me to have ferocious headaches) and be monitored very frequently. She was born with one extra beat every 5, then it decreased to one extra beat every 20. Eventually, the arrhythmia disappeared.
Arrhythmia is not unusual among newborns and it usually works itself out as soon as the heart "teaches" itself what its proper rhythm should be. However, I would have liked to have been sufficiently warned about the possible correlation of coffee with a racing heartbeat in my baby.
"There's no food in your food!!" Joan Cusack to John Cusack in "Say Anything."