While I personally like babies and children and have very strong beliefs they should be loved and cared for, I have long been concerned re: global overpopulation of the human species and I am rarely thrilled when someone procreates. I remember the 1970s and the "Population Bomb.” At that time it was acceptable for people to have these discussions, and it was more socially acceptable for people to openly choose not to have children for themselves and the planet. I can never understand why one doesn't choose to adopt or choose not to have a child at all.
Is there a polite way for me to express my disapprobation, or at the very least to convey my belief we should be working to make fewer people, rather than more?
Etiquetteer has thought carefully about how one might raise this issue with new or expectant parents, and has come to the conclusion that any suggestion they shouldn't be having their child would not be received well. Perhaps they could misinterpret it to think that you were suggesting they should end the pregnancy, which Etiquetteer feels sure you don't mean. Etiquetteer is curious to know what reaction you might expect. To use a baker's euphemism, there's no use arguing about the recipe once the bun's in the oven.
At the very least, since you appear knowledgeable about world population issues, you might in a general way share current data or news items. (Certainly things are getting interesting in China since they lifted the One Child Only limit.)
But be careful. One of those expectant parents could take offense and suggest that you yourself assist the problem of overpopulation by eliminating yourself from the population, which Etiquetteer would not like to see.