1. Conceiving is more difficult for obese women; women shaped like pears have more fertility luck than apple-shaped women with a high waist-hip ratio.
2. Dieting when your child is in the womb isn't good for mom or baby. Losing weight may leave you low on calories and carbohydrates, iron, folic acid, and other important vitamins and minerals.
3. Diseases such as hypertension, gestational diabetes, and blood clots in the veins in your legs abound for rounder moms.
4. Overweight women pay a price, with higher prenatal medical costs from hospitalizations and medicines needed to treat related diseases and other health problems.
5. Bigger moms make bigger babies. Giving birth to larger babies isn't easy, labor lasts longer because they don't easily fit down the birth canal.
6. Weight gain between pregnancies doubles your risk of having a baby that weighs more than the average seven-pound baby. At delivery time, babies with heavier moms weigh about 8 pounds, 13 ounces.
7. Overweight mothers are more likely to need a cesarean delivery.
8. Babies of heavier moms are more likely to be admitted to neonatal intensive care units. They suffer from a higher incidence of defects in the brain and spinal cord, such as spina bifida.
9. More mommy weight often means more time spent recuperating from childbirth in the hospital. Because overweight women suffer more complications, such as cesarean sections, they need more recovery time.
10. Larger babies grow up to be bigger kids and heavier adults. Being overweight increases your children's risk of heart disease, stroke, and diabetes, as well as other diseases and health conditions.

Sources:

British Medical Journal

Fred Hutchinson Cancer Research Center

Dr. Anthony Scialli, Public Affairs Committee of the Teratology Society

St. Louis University