Could the following be considered a reason for a judge to void a parenting plan or at least modify it?

You don't say when any of this occurred nor give information except that you were under duress at the time. You will have to be more specific as to the duress-Was it duress because you were emotionally in a turmoil due to the pregnancy or because you were being threatened? Did you have an attorney at the time? How old are the children now? Either of you remarried? Courts are very reluctant to ever void a Parenting Plan because what would happen to the children? They may or may not modify a plan but you have to have a material change in circumstances. You seem to be saying that the plan should be changed because of your emotional situation and only someone with more information could tell you the odds of success with that. I know that in my state, depending on the time since the entry, it is unlikely that your emotional status as the time, absent more, would be sufficient to modify a plan Talk to a lawyer in a state where the plan was entered. They would have a better ability to tell you things.

It is very difficult to set aside a signed Marital Settlement Agreement. At best, it could be modified but to modify the Parenting Plan (which I believe you are referring to and that you probably entered into) as it relates to children you have to show a substantial, material and unanticipated change of circumstances that affects the best interests of the children. That standard is really an uphill burden for the parent wishing to modify the parenting plan but of course it should be done. You should contact an experienced family law attorney well versed in child custody and parenting plan modifications in deciding how to proceed.