Impact on the environment is reasonably attributed to the size of the population times the amount consumed/ produced times the content and methods used in consumption/ production/ waste disposal. All three contribute, none contribute independently of the others. Whether or not the US is at replacement fertility doesn’t change that. Whether or not someone wants to consider this in making a highly personal decision such as whether or not to have biological children is of course their choice. But I was struck that this implication of childbearing was not mentioned in the initial discussion.