It isn't just the US, every modern industrial age society is having the same declines. Japan has a negative growth rate, so does China and many other S.E Asian countries. All of the European countries have very low birth rates as well.
At the rate that industrialized nations are dropping they are all going to have to resort to immigration to keep a balanced or growing population.
As for why, there are some known factors, people decide that kids are expensive and so avoid having them, women want a career and put off having them, an ever growing percentage of people are too obese to effectively get pregnant.
There is also our diet which in modern industrialized nations is horrific, our food is full of toxins and poisons so that it is easily packaged boxed and will last for a long time on a shelf. I see other families eating and often comment on the fact that I don't actually see anything that I would define as food in their diet. On road trips I find myself walking through the gas station trying to find something that actually resembles food, I generally give up, there is nothing actually edible or food like being sold at those places.
The wife and I have had six kids, we are both plenty fertile, but then again we don't drink pop, beer, fruity drinks, energy drinks or any of the other crazy crap out there. We stick to some home brewed coffee, some home brewed tea, milk, well water and some home brew beer and mead. For food we avoid anything that has been processed or pre made, everything we eat is made from scratch each meal. We slaughter and process our own meats and grow and or obtain much of the fruit and vegetables that we eat as well. We are reasonably active, though the wife is paraplegic and stuck in a wheelchair and about 2 out of every 3 years she is stuck on bed rest, so "she" is not real active.
The 4 kids still at home are now all teenagers and in sports, none of their coaches has believed them when they were asked how much they weigh, they literally look like toothpicks. My youngest Sierra is 13 standing in at 5ft4 and weighs in at 125 pounds. My next youngest John is 15 standing at 5ft 11 1/2 and weighs in at 165, his coaches had to use three scales before they would believe his weight they were convinced that he couldn't weigh more than 125. Jarad at 17 is 5ft 8 and weighs in at 165 as well. Ted is 18 and is 5ft5 and weighing in at 155. All of them like I said look like toothpicks compared to the other kids but they are the same weight and are pound for pound much stronger. The food modern families are eating is really screwing kids up, the kids should like mine have much more lean muscle mass and much less fat tissue. My son John was the only kid in the HS to make state in cross country this year, the funny part of that is that he has been doing cross country and football at the same time and he is now going to state in football as well. The only difference between my kids and the others is their diet, I am convinced that much of the problem with todays youth comes down to not having proper food.
