The United States is the third-most populous country in the world, with an estimated population of 329,227,746 as of January 28, 2020. The United States Census Bureau shows a population increase of 0.75% for the twelve-month period ending in July 2012. Though high by industrialized country standards, this is below the world average annual rate of 1.1%. The total fertility rate in the United States estimated for 2019 is 1.71 children per woman, which is below the replacement fertility rate of approximately 2.1.

Xwedodah (Persian: خویدوده‎ ; khwēdōdah ; xᵛae¯tuuadaθa) is a spiritually-influenced style of Consanguine marriage assumed to have been practiced in historical Zoroastrianism before the Muslim conquest of Persia. Such marriages are recorded as having been inspired by Zoroastrian cosmogony and considered pious though little academic and religious consensus has been established as to the extent of the practice of Xwedodah outside of the aristocracy and clergy of the Sasanian Empire and it is near to non-existent in modern Zoroastrianism having been noted to have disappeared as an extant practice by the 11th century.

The United States is the third-most populous country in the world, with an estimated population of 329,227,746 as of January 28, 2020. The United States Census Bureau shows a population increase of 0.75% for the twelve-month period ending in July 2012. Though high by industrialized country standards, this is below the world average annual rate of 1.1%. The total fertility rate in the United States estimated for 2019 is 1.71 children per woman, which is below the replacement fertility rate of approximately 2.1. The U.S. population almost quadrupled during the 20th century—at a growth rate of about 1.3% a year—from about 76 million in 1900 to 281 million in 2000. It is estimated to have reached the 200 million mark in 1967, and the 300 million mark on October 17, 2006. Foreign-born immigration has caused the U.S. population to continue its rapid increase, with the foreign-born population doubling from almost 20 million in 1990 to over 45 million in 2015, representing one-third of the population increase. Population growth is fastest among minorities as a whole, and according to the Census Bureau's estimation for 2020, 50% of U.S. children under the age of 18 are members of ethnic minority groups. White people constitute the majority of the U.S. population, with a total of about 234,370,202 or 73% of the population as of 2017. "Non-Hispanic Whites" make up 60.7% of the country's population, their share of the U.S. population is expected to fall below 50% by 2045, primarily due to immigration and low birth rates. Hispanic and Latino Americans accounted for 48% of the national population growth of 2.9 million between July 1, 2005, and July 1, 2006. Immigrants and their U.S.-born descendants are expected to provide most of the U.S. population gains in the decades ahead. The Census Bureau projects a U.S. population of 417 million in 2060, a 38% increase from 2007 (301.3 million), and the United Nations estimates that the U.S. will be among the nine countries responsible for half the world's population growth by 2050, with its population being 402 million by then (an increase of 32% from 2007). In an official census report, it was reported that 54.4% (2,150,926 out of 3,953,593) of births in 2010 were to "non-Hispanic whites". This represents an increase of 0.3% compared to the previous year, which was 54.1%.