M.J.
I posted this on another thread, but this may be relevant to you as well.
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In today's world, two is ample.
I was listening to the Diane Rehm show about "World Population" and America's the most over populated country based on the resources we consume. We have 6.8 billion people in the world, we're losing the biodiversity in the planet, running out of water and other critical resources. Environmental sustainability depends on how many people there are, how quickly the population is growing etc. The panelist also said that ethically we should not have more than 2 children since its going to affect every body else.
A wonderful alternative is available... have you considered adoption?
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Plus, I think that not having the time to bond with your children because they were twins may not be a real reason to have another child. You'll likely have even less time to spend with your twins if you have a third. And coming to other important aspects, will you be able to provide them each with a good college education at the very least ? In 2028, a college education will cost about $200K-$300K per child. With 3 kids, multiply that number.