Who’s the 300 Millionth?

Valerio Mayte Perez yawns a few hours after her birth, on the day America’s official population passed the 300-million mark.  (Nick Ut / AP)

Sonia Bosque photographs her baby girl Genesis, born the day the U.S. Census Bureau estimated U.S. population at 300 million.  (Roberto Schmidt / AFP/Getty Images)

Anareli Meza, held here by her mother Catalina Meza, entered the world at 5:10 a.m. — 24 minutes after the moment the Census Bureau had estimated the United States population would reach 300 million.  (Robyn Beck / AFP/Getty Images)

The focus was on newborns, but the 300-millionth American could just as easily be an immigrant. Pedestrians pass fortified border fences as they cross from Mexico to the United States.  (David McNew / Getty Images)

Cars queue up Tuesday to go from Tijuana, Mexico, to San Ysidro, California, at the world’s busiest border crossing.  (David McNew / Getty Images)

Pedestrians cross the border from Mexico to the U.S. on the morning the official U.S. population estimate hit 300 million.  (David McNew / Getty Images)

U.S. Census Bureau employees in Suitland, Md., clap Tuesday morning for the estimated 300-millionth American.  (Mark Wilson / Getty Images)

A display of the national population estimate at the U.S. Census Bureau in Suitland, Md., hits 300,000,000 Tuesday morning.  (AP / APTV)