The past year was rich in news: war, politics, death and destruction in East Asia, among other things. But I am also going to remember 2004 as the year I was invited to watch my eldest daughter’s sonogram.

These days, the use of ultrasound technology to check out an expected child isn’t unusual. I had seen the printouts from her first pregnancy and from our second daughter’s pregnancies as well. Everybody oohs and ahhs obligingly at such images, but frankly they aren’t all that appealing.

But actually watching the images of a four month-old baby squirming around inside the womb and sucking on its thumb brings a different level of appreciation. Wow, this is a child! Not a fetus, not a clump of tissue, not some alien-looking creature, but a real live human being getting ready to take its place in the world.

But after the emotional impact subsided a bit, another thought intruded in the doting grandpa’s incorrigible policy-wonkish mind: this is child No. 2 – one-tenth of a child shy of what the demographers tell us is the “replacement rate” for a society such as ours. Two children are enough to replace the parents, but if the broader social goal is to maintain population in a steady state, more are needed. Even in a modern society, some children either can’t or won’t reproduce themselves.

Our daughter and her husband will make their own decisions, of course. And the grandparents can hardly complain: Two is the number my wife and I settled on, and that has certainly worked out well for us. The average American family apparently agrees. The American fertility rate is now exactly 2.0, nearly half the rate only a few decades ago. In the days when a majority of the population worked the land and death rates were higher, families usually picked a far higher number. But things are different now – thank goodness.

Odd, then, that there is still such hysteria about population growth. As recently as the early 1970s, a learned Stanford professor, Paul Ehrlich, could pen a bestseller titled “The Population Bomb,” predicting mass starvation as early as the 1980s as a result of over-population. Environmentalists fretted about mass die-offs of other species at the ravening hands of humans.

But the bomb never went off, and not just because of birth control. Human ingenuity was providing more food and wealth than ever. The problem in most parts of the world is not shortages but surpluses. And while the alarmists have been slow to pick up on the fact – visions of apocalypse are good for fund-raising – fertility rates have been falling dramatically.

One of the more interesting books of 2004 was “Fewer: How the New Demography of Depopulation Will Shape Our Future,” by Ben Wattenberg, a senior fellow (and longtime Democrat) at the American Enterprise Institute in Washington. As Mr. Wattenberg notes, the fertility rate in Western Europe and Japan has sunk to 1.8 from 2.7 in the early 1950s, meaning those areas are starting to lose population. Fertility rates are even plunging in the less developed countries of South America, Africa, and most of Asia: to about 2.7 from about 6.0 only a few decades ago.

Still worried about swarms of Mexicans pouring over the border? Their fertility rate is down to a modest 2.5 from 6.9 in the late 1950s.

Many will see this as good news, and maybe it will turn out that way. But it could also mean fewer Einsteins – and even fewer workers to pay for the Social Security many Americans think they have been promised. As Mr. Wattenberg says, zero growth in population, much less a decline, raises serious questions. The plain fact of the matter is that the last few centuries of population explosion have also been a time of accelerating, worldwide prosperity.

It’s hard, in fact, to resist the thought that that little image on the hospital monitor, who will soon take his or her place in the outside world (the parents didn’t want to know), is sending another message: Happy New Year. When you see such a thing, it’s much harder to view population as a liability rather than an asset.

Mr. Bray is a Detroit News columnist.