if you gain an excessive amount of weight while pregnant, what harmful effects could this have on the fetus?

The link below has some information on it for you. Basically, the bigger you get, often, the bigger the baby gets, which mainly means vaginal delivery may be difficult or impossible, needing a caesarean, or if you do deliver vaginally, the baby could have problems from it like shoulder dystocia.

I think excess weight is also associated with neural tube defects to some degree, as well as diabetes.

Your risk of gestational diabetes as well as the chance of developing high blood pressure go up considerably. Both can be harmful to you and your unborn child. Premature birth, extremely large baby, chance for c-section and in rare cases fetal death.

Your weight is on the baby if you lay down or something. and it could cause the baby to be huge and it could cause you to have gestational diabetes. Just be careful not to gain lot more weight.

If you are gaining too much weight that means you are not eating healthy then your baby will not get needed nutrients to healthy development. And yes Gestational diabetes.