AMERICAN BIRTH RATES seem to be moving upward again -- slightly. This tenative reversal of the long decline will bring increased sales of diapers this year and a modest rise in elementary school enrollments in the fall of 1985. It will be good for the automobile business around the year 2000 -- assuming that there's still an automobile business then -- and good for the residential real estate market shortly thereafter.

On the other hand, none of that will make it any easier to find parking spaces in 2001. It's something of a setback to the movement to curb population growth. The death rates fell a little last year, once again, and there were 1.7 million more births than deaths.

There were 4.3 million babies born in 1960. By 1973, the number had fallen to 3.1 million. But last year it was back up to 3.5 million births. One reason is that there are more young women in the population. But there's more to it than that. The best measure of the trend is the number of births in relation to the number of women of childbearing age. This went very high 25 years ago, then dropped precipitately to barely half that level in the mid-1970s. For the past four years, it's been moving up a little and down a little -- but mostly up.

Is that in your own interest? It means more children to be educated at public expense -- and there go the property taxes again. It probably means more street crime in the late 1990s; street crime tends to rise in proportion of young males in the population. But it also means more productive workers by the turn of the century. They, incidentally, will be paying Social Security taxes to provide the pension checks for those people, now in their 40s, who will be retiring then.

The change in the trend indicates that some of the ideas of the early 1970s about personal freedom and fulfillment may be changing a bit. More women are older when they have their first children, then in the past. Apparently some of the women who decided against children a decade ago have reconsidered. No one ever knows entirely why birth rates change.The statistics only let you guess. Birth rates begin with the most private of choices -- although they carry vast public consequences.