Measuring Fertility in the United States — Penn Wharton Budget Model

Summary: The U.S. population’s total fertility rate is now about 1.7 births per woman, which is below the replacement rate of 2.1 needed for the U.S. population not to shrink without increased immigration. Women are delaying motherhood, from the 2006 average age group of 25 to 29 to the 30 to 34 age group today.


Birth rates affect the potential size of the labor force, GDP, debt and other macroeconomic indicators. Demographers and economists measure fertility using different metrics. Each measure has its advantages depending on the research focus. This post reports fertility trends from 2006 to 2019 for several of the most prominent measures. Data comes from the American Community Survey (ACS).

Measures of fertility

The Age Specific Fertility Rate (ASFR) measures the annual number of births to women in a specific age cohort (typically a five-year age cohort, eg 24 to 29) per 1,000 women in that cohort. This measure relates the actual number of births to an age cohort within a year. The ASFR is useful for analyzing life-cycle patterns of fertility, but cannot be used to track changes in the total fertility rate because the underlying population’s age distribution changes over time.

Figure 1: Age-specific fertility rates (ASFRs) over time


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Figure 1 plots the ASFR of women aged 15 to 49 in five-year age cohorts over the years 2006 to 2019. Teen pregnancies have declined from a peak of 28 births per 1,000 women aged 15 to 19 in 2007 to less than 11,000 births as of 1, 2019. Fertility peaked in the 25 to 29 age group from 2006 to 2011, but the peak of fertility rose to the 30 to 34 age group. Over this period, the number of births also increased for the three oldest cohorts: for example, the average number of births per 1,000 women aged 40-44 rose by 40 percent from 15 to 21 births per 1,000 women. This suggests that women are delaying fertility and having children at older ages than was historically the case.

The Total Fertility Rate (TFR) is the number of children a woman would have if she progressed instantaneously through all fertile years with the given ASFR at each age.1 The TFR is a theoretical measure of fertility that gives an idea of ​​what fertility looks like on an annual basis. The calculation of the TFR requires only a single year of data and provides a reasonable comparison of fertility over years. However, the TFR does not reflect completed fertility over a woman’s lifetime and can be skewed by timing of changes in fertility. For example, if women in a certain age cohort delay childbearing, that delay will show up as a decline in TFR when that cohort is early in their childbearing years, ceteris paribus.

The TFR is typically the measure used in discussing a population’s replacement rate, the rate required to maintain a population’s current size, without considering any potential migration effects. The concept of the replacement rate has been in place since about 1930 and is equal to about 2.1 for the US (as for most developed countries). A number above 2.1 is associated with a growing population, and anything below 2.1 indicates population decline.

Figure 2 plots the calculated Total Fertility Rate (TFR) from 2006 to 2019. The TFR rose to around 2.2 births per woman between 2007 and 2008, but fell below replacement level in 2010, and further declined to 1.7 in 2019.

The General Fertility Rate (GFR) is a ratio that measures the number of births per 1,000 women of childbearing age. Like the TFR, the GFR provides a good picture of current fertility or fertility within a given year. Unlike the TFR, the GFR is not age specific and is a measure of actual births. The GFR has the advantage of being easy to explain, but it is largely driven by changes in the underlying age structure of the population. For example, women in their 40s have very few children. Thus, as the share of the female population over 40 increases, the GFR will decrease even if the ASFRs and TFR do not change.

Figure 3 plots the calculated General Fertility Rate (GFR) in births per 1,000 women from 2006 to 2019. The GFR followed a similar pattern to the TFR, rising in 2008 when it reached 60 births per 1,000 women of childbearing age did, and then took off. In 2019, births decreased to 51 per 1,000 women of childbearing age.

The Complete Fertility Rate (CFR) measures the number of children women have had over their lifetime. This measure can only be calculated for cohorts of women who are older than childbearing years. CFRs provide the most accurate measure of lifetime fertility, but are not useful in calculating current fertility rates, which are determined by women still of childbearing age. We do not calculate CFRs because recent ACS surveys do not ask women how many children they have ever had.

This analysis was conducted by Maddison Erbabian and Victoria Osorio. Prepared for the website by Mariko Paulson.

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