From his post on America and Life Expectancy

The United States counts all births as live if they show any sign of life, regardless of prematurity or size. This includes what many other countries report as stillbirths. In Austria and Germany, fetal weight must be at least 500 grams (1 pound) to count as a live birth; in other parts of Europe, such as Switzerland, the fetus must be at least 30 centimeters (12 inches) long. In Belgium and France, births at less than 26 weeks of pregnancy are registered as lifeless. And some countries don’t reliably register babies who die within the first 24 hours of birth. Thus, the United States is sure to report higher infant mortality rates.

The other factor here is that thanks to our access to medical technology, we’re more likely to try to save premature deliveries that in other countries would result in stillbirths or miscarriages. So every time an infant dies in the U.S. that would never have been born alive (or counted as born alive) in other countries, it registers as a life that died at the age of “zero.” That’s a pretty significant downward-tug on the national life expectancy.

…

I’d actually like to see where we rank in average life expectancy from, say, the age of 30 or 35 onward. I couldn’t find any such data, but it seems to me that would factor out much of the homicide problem, would negate the problems with how we measure infant mortality, and would probably result in a better showing for the U.S.

All quite true.