Obese pregnancies have a much higher risk of miscarriage, all sorts of defects, stillbirth.  Plus they're pretty much guaranteed to get gestational diabetes and have a giant sugar baby.  You should eat healthy while you're pregnant but your body will leech nutrients from your stores to get what it needs.  That's why it's best to wait 2 years+ between children because you could end up with brittle bones from calcium leeching.