Birthweight and preterm birth

Of the babies born in 2018 to 2024 in the least deprived tenth of large American counties, 7.4%7.4% percent of live births under 2,500 gramsEstimandbirths-weighted mean of county shares (the pooled share), bootstrap interval over countiesInterval95% CI 7.2% to 7.7%n57 countiesWeightsbirths in each countyPopulationlive births in the least deprived tenth of the named counties by county deprivation rankPeriodbirths in 2018 to 2024SourceCDC WONDER natality weighed less than 2,5002,500 gramsEstimandbirthweight below which a live birth counts as low birthweight (a definition)Weightsnone (definition)Populationlive birthsPeriodall years (a definition)SourceCDC WONDER natality grams, the weight below which a newborn counts as low birthweightLow birthweightA live birth weighing less than 2,500 grams, the cutoff used in vital statistics.. In the most deprived tenth of the same counties the share was 9.7%9.7% percent of live births under 2,500 gramsEstimandbirths-weighted mean of county shares (the pooled share), bootstrap interval over countiesInterval95% CI 9.2% to 10.3%n58 countiesWeightsbirths in each countyPopulationlive births in the most deprived tenth of the named counties by county deprivation rankPeriodbirths in 2018 to 2024SourceCDC WONDER natality. A large county here is one of 100,000100,000 residentsEstimandpopulation size at which WONDER names a county (a definition)Weightsnone (definition)Populationcounties that CDC WONDER natality names individuallyPeriodbirths in 2018 to 2024SourceCDC WONDER natality or more residents, the size at which the CDC WONDER birth records name a county on its own. Birthweight and the length of pregnancy are both recorded on the birth certificate, which makes them the standard early measures of how a pregnancy went.

Low birthweight and preterm birth rise with county deprivation

Deprivation here is a rank given to places. Each census tract has a national rank on the Area Deprivation IndexArea Deprivation Index (ADI)A score that ranks neighborhoods by income, education, employment and housing conditions drawn from the American Community Survey. In this book a higher national percentile means a more deprived neighborhood., which orders the country's neighborhoods by income, education, employment and housing measures from the American Community Survey.[1] A county's rank is the average of its tracts' ranks, weighted by population, and the large counties are then split into tenths that hold equal numbers of counties. Each tenth's rate pools the births in its counties, so it describes counties and groups of births, not individual mothers.

Low birthweight rises at every step from the least deprived tenth to the most deprived, a difference of 2.3 percentage points (95% CI 1.7 percentage points to 2.9 percentage points)2.3 percentage points percentage pointsEstimanddifference of two births-weighted decile means of the low-birthweight share, bootstrap interval over countiesInterval95% CI 1.7 percentage points to 2.9 percentage pointsn578 countiesWeightsbirths in each countyPopulationlive births in the named counties, most deprived tenth minus least deprived tenthPeriodbirths in 2018 to 2024SourceCDC WONDER natality between the two ends. Preterm birthPreterm birthA birth before 37 weeks of pregnancy., a birth before 3737 completed weeks of gestationEstimandgestational age below which a live birth counts as preterm, obstetric estimate (a definition)Weightsnone (definition)Populationlive birthsPeriodall years (a definition)SourceCDC WONDER natality weeks of pregnancy, follows the same path, from 8.8%8.8% percent of live births before 37 weeksEstimandbirths-weighted mean of county shares (the pooled share), bootstrap interval over countiesInterval95% CI 8.6% to 9.0%n57 countiesWeightsbirths in each countyPopulationlive births in the least deprived tenth of the named counties by county deprivation rankPeriodbirths in 2018 to 2024SourceCDC WONDER natality of births in the least deprived tenth to 12.1%12.1% percent of live births before 37 weeksEstimandbirths-weighted mean of county shares (the pooled share), bootstrap interval over countiesInterval95% CI 11.3% to 12.7%n58 countiesWeightsbirths in each countyPopulationlive births in the most deprived tenth of the named counties by county deprivation rankPeriodbirths in 2018 to 2024SourceCDC WONDER natality in the most deprived. The two measures overlap, since a baby born early is usually small, though a baby can also be small at full term.

Figure 1

Low birthweight rises from 7.4% of births in the least deprived tenth of large counties to 9.7% in the most deprived

Share of live births under 2,500 grams (and before 37 weeks in the second tab) by county deprivation, 2018 to 2024

Low birthweight, large countiesPreterm and low birthweightAll counties

Preterm birth is more common than low birthweight in every decile and rises more, from 8.8% to 12.1%

Births-weighted share of live births before 37 weeks and under 2,500 grams by decile of county deprivation, 2018 to 2024

Decile of county deprivation rank (1 = least deprived tenth of counties, 10 = most deprived)

Both lines rise at every step. The bands are 95% intervals from a bootstrap over counties.

Show data

Data for Low birthweight rises from 7.4% of births in the least deprived tenth of large counties to 9.7% in the most deprived, Preterm and low birthweight

conf_high	conf_low	decile	estimate	measure	n
9.01	8.59	1	8.82	Preterm birth	4,151,203
7.68	7.18	1	7.43	Low birthweight	4,150,007
9.48	9.08	2	9.28	Preterm birth	2,647,866
8.00	7.32	2	7.63	Low birthweight	2,646,210
10.12	9.47	3	9.82	Preterm birth	2,523,259
8.36	7.55	3	7.96	Low birthweight	2,522,181
10.29	9.80	4	10.05	Preterm birth	1,825,582
8.67	7.84	4	8.29	Low birthweight	1,824,679
10.53	9.98	5	10.27	Preterm birth	1,923,159
8.65	7.97	5	8.37	Low birthweight	1,922,260
11.28	10.26	6	10.80	Preterm birth	2,196,492
8.98	8.29	6	8.69	Low birthweight	2,195,346
11.22	10.56	7	10.92	Preterm birth	1,520,420
9.50	8.57	7	9.05	Low birthweight	1,519,471
11.59	10.92	8	11.27	Preterm birth	1,469,109
9.85	8.89	8	9.40	Low birthweight	1,468,025
11.82	11.01	9	11.42	Preterm birth	1,269,566
10.18	9.00	9	9.63	Low birthweight	1,268,228
12.66	11.32	10	12.05	Preterm birth	891,594
10.26	9.20	10	9.68	Low birthweight	890,018

How we know

The counts come from CDC WONDER natality for births in 2018 to 2024, in the 578578 countiesEstimandcount of countiesWeightsnone (full count)Populationcounties that WONDER natality names individually and that have birthsPeriodbirths in 2018 to 2024SourceCDC WONDER natality counties that WONDER names individually. Those counties hold 79.5%79.5% percent of live births in the United StatesEstimandshare of births in the named countiesn25,709,900 birthsWeightsnone (full count)Populationlive births in the 50 states and DCPeriodbirths in 2018 to 2024SourceCDC WONDER natality of births in the United States. WONDER pools the births of every other county in a state into one row, so those counties cannot be placed on the deprivation scale. The most deprived tenth here is therefore relative to large counties and not to the nation. Where WONDER withholdsSuppressionThe withholding of a number because the count behind it is too small to publish without risking the identification of a person. A suppressed value is missing, not zero. a cell of births, the chapter treats it as missing, leaving it out of both the count and the denominator, and never reconstructs it. Withheld cells are not spread evenly. Those in the birthweight cut could hold at most 1.3%1.3% percent of live birthsEstimandlargest share of births the withheld birthweight cells could hold: 9 births in each withheld cell, over all births in these countiesWeightsnone (an upper limit from the cell count)Populationlive births in the most deprived tenth of the named countiesPeriodbirths in 2018 to 2024SourceCDC WONDER natality of births in the most deprived tenth and 0.2%0.2% percent of live birthsEstimandlargest share of births the withheld birthweight cells could hold: 9 births in each withheld cell, over all births in these countiesWeightsnone (an upper limit from the cell count)Populationlive births in the least deprived tenth of the named countiesPeriodbirths in 2018 to 2024SourceCDC WONDER natality in the least deprived. The published difference between the tenths could therefore be too small by up to the first amount and too large by up to the second. County Health Rankings, which takes its birth counts from the same national vital statistics system, publishes county rates for births in 2016 to 2022 that include small counties. In tenths of all counties with a published rate, the line rises over the first tenths, is flat across the middle ones and rises again at the top (Figure 1, “All counties”). The climb is therefore steadier across large counties than across the whole country. The report gives every estimate with its interval and sample size.

The gap by mother's race, at every level of deprivation

The gradient averages over all the mothers in a county. County Health Rankings also publishes rates by the mother's race and ethnicity, counting mothers as non-Hispanic Black, non-Hispanic white or Hispanic of any race. Pooled over the large counties where a rate is published, low birthweight was 14.0%14.0% percent of live births under 2,500 gramsEstimandpooled share of births, from the published county rates and the births they imply, bootstrap interval over countiesInterval95% CI 13.8% to 14.2%n549 countiesWeightsbirths to mothers of the group in each countyPopulationlive births to non-Hispanic Black mothers in the named counties where County Health Rankings publishes the ratePeriodbirths in 2016 to 2022SourceCounty Health Rankings of births to non-Hispanic Black mothers and 6.7%6.7% percent of live births under 2,500 gramsEstimandpooled share of births, from the published county rates and the births they imply, bootstrap interval over countiesInterval95% CI 6.6% to 6.8%n578 countiesWeightsbirths to mothers of the group in each countyPopulationlive births to non-Hispanic white mothers in the named counties where County Health Rankings publishes the ratePeriodbirths in 2016 to 2022SourceCounty Health Rankings of births to non-Hispanic white mothers, for births in 2016 to 2022. That gap, 7.3 percentage points (95% CI 7.0 percentage points to 7.4 percentage points)7.3 percentage points percentage pointsEstimanddifference of pooled low-birthweight shares, county-cluster bootstrap intervalInterval95% CI 7.0 percentage points to 7.4 percentage pointsn578 countiesWeightsbirths to mothers of the group in each countyPopulationlive births in the named counties, non-Hispanic Black mothers minus non-Hispanic white mothersPeriodbirths in 2016 to 2022SourceCounty Health Rankings, is larger than the difference of 2.3 percentage points2.3 percentage points percentage pointsEstimanddifference of two births-weighted decile means of the low-birthweight share, bootstrap interval over countiesInterval95% CI 1.7 percentage points to 2.9 percentage pointsn578 countiesWeightsbirths in each countyPopulationlive births in the named counties, most deprived tenth minus least deprived tenthPeriodbirths in 2018 to 2024SourceCDC WONDER natality between the most and least deprived tenths of counties, which comes from WONDER births in 2018 to 2024. Infant deaths follow the same order. WONDER's linked birth and infant death records carry the mother's race only for 2018 and 2019, as a single race of any ethnicity. Their white group therefore includes Hispanic mothers and is not the group in the low-birthweight rates. Infant mortality, the outcome that follows these births, is the subject of Chapter 2.2, Infant mortality.

Figure 2

Non-Hispanic Black mothers had low-birthweight births at 14.0% against 6.7% for non-Hispanic white mothers, a ratio of 2.1

First panel: share of live births under 2,500 grams in the large counties, by mother's race and ethnicity, County Health Rankings, births in 2016 to 2022. Second panel: infant deaths per 1,000 live births in the United States by mother's single race, any ethnicity, 2018 and 2019

Low birthweight (% of live births)

Infant mortality (deaths per 1,000 live births)

Percent of live births (low birthweight panel) or deaths per 1,000 live births (infant mortality panel)

Show data

Data for Non-Hispanic Black mothers had low-birthweight births at 14.0% against 6.7% for non-Hispanic white mothers, a ratio of 2.1

conf_high	conf_low	label	n	panel	race	value
14.19	13.75	Black (non-Hispanic)	3,249,387	Low birthweight (% of live births)	Black	13.98
7.70	7.39	Hispanic (any race)	5,632,060	Low birthweight (% of live births)	Hispanic	7.54
6.82	6.64	White (non-Hispanic)	9,614,256	Low birthweight (% of live births)	White	6.73
10.49	10.14	Black (any ethnicity)	1,255,629	Infant mortality (deaths per 1,000 live births)	Black	10.32
4.78	4.66	White (any ethnicity)	5,617,093	Infant mortality (deaths per 1,000 live births)	White	4.72

Counties differ in deprivation, so the race gap could shrink once counties are compared at the same level. The chapter splits the large counties into four groups of equal size by deprivation rank, called quartiles. In every quartile, the rate for non-Hispanic Black mothers is higher than the rate for non-Hispanic white mothers, for births in 2016 to 2022. The rate for non-Hispanic Black mothers in the least deprived quartile, 12.4% (95% CI 11.9% to 12.8%)12.4% percent of live births under 2,500 gramsEstimandpooled share of births, bootstrap interval over countiesInterval95% CI 11.9% to 12.8%n133 countiesWeightsbirths to mothers of the group in each countyPopulationlive births to non-Hispanic Black mothers in the least deprived quartile of the named counties by deprivation rankPeriodbirths in 2016 to 2022SourceCounty Health Rankings, is higher than the rate for non-Hispanic white mothers in the most deprived quartile, 7.6% (95% CI 7.5% to 7.7%)7.6% percent of live births under 2,500 gramsEstimandpooled share of births, bootstrap interval over countiesInterval95% CI 7.5% to 7.7%n145 countiesWeightsbirths to mothers of the group in each countyPopulationlive births to non-Hispanic white mothers in the most deprived quartile of the named counties by deprivation rankPeriodbirths in 2016 to 2022SourceCounty Health Rankings.

Figure 3

Low birthweight is more common for non-Hispanic Black mothers in the least deprived quartile of counties (12.4%) than for non-Hispanic white mothers in the most deprived (7.6%)

Low birthweight (share of live births under 2,500 grams, births in 2016 to 2022) or infant mortality (deaths per 1,000 live births, 2018 and 2019) by quartile of county deprivation rank and mother's race

Low birthweightInfant mortality

Births under 2,500 grams (% of live births)

• Black (non-Hispanic)
• White (non-Hispanic)
• White (non-Hispanic), most deprived quartile

Quartile of counties by deprivation rank

Show data

Data for Low birthweight is more common for non-Hispanic Black mothers in the least deprived quartile of counties (12.4%) than for non-Hispanic white mothers in the most deprived (7.6%), Low birthweight

conf_high	conf_low	counties	measure	n	quartile	race	rate
12.76	11.95	133	lbw	830,778	Least deprived quartile	Black (non-Hispanic)	12.37
6.34	6.05	144	lbw	3,597,717	Least deprived quartile	White (non-Hispanic)	6.18
14.02	13.46	137	lbw	862,783	Second quartile	Black (non-Hispanic)	13.76
6.75	6.53	145	lbw	2,307,987	Second quartile	White (non-Hispanic)	6.64
14.78	14.14	139	lbw	823,241	Third quartile	Black (non-Hispanic)	14.46
7.23	7.05	144	lbw	2,156,968	Third quartile	White (non-Hispanic)	7.14
15.80	15.25	140	lbw	732,585	Most deprived quartile	Black (non-Hispanic)	15.53
7.70	7.45	145	lbw	1,551,584	Most deprived quartile	White (non-Hispanic)	7.58

What the county comparison shows and does not show

These are patterns among places. A county's rank averages over neighborhoods that differ. A Black mother in a county ranked among the least deprived may live in a neighborhood more deprived than the county average. A white mother in a county ranked among the most deprived may live in a less deprived one. The comparison therefore cannot show that mothers under the same neighborhood conditions have different outcomes. What it shows is that sorting counties by deprivation does not remove the gap.The weatheringWeatheringThe hypothesis that chronic stress from disadvantage and discrimination wears down health faster, so that the same outcomes appear at younger ages in affected groups. Arline Geronimus proposed it in studies of birth outcomes. hypothesis proposed by Geronimus holds that the health of Black women may begin to deteriorate in early adulthood as a physical consequence of cumulative socioeconomic disadvantage.[2] A gap that persists at every level of county deprivation is consistent with it, but a comparison across counties cannot separate it from other explanations. A comparison of Black and white children raised in the same tract is in Chapter 4.3, Race, income and mobility. A study of federal expansions of the Earned Income Tax CreditEITCThe Earned Income Tax Credit, a refundable federal tax credit for low and moderate income workers. It grows with earnings up to a limit and then phases out. attributes a lower rate of low birthweight among mothers who received larger credits to the added income.[3] Its estimates apply to the policies and years studied, and they do not measure how much of the county gradient or the race gap such a policy would close.

What this cannot tell you

A pattern across counties does not describe the babies in any one of them, and a county ranked as least deprived can hold deprived neighborhoods. The Area Deprivation Index also understates deprivation in high-cost cities (Chapter 1.1, What the deprivation index measures). The index, the WONDER births and the rates by mother's race cover different years, and the intervals leave those differences out. No comparison here identifies a mechanism, since deprivation and race are both tied to smoking, prenatal care, chronic stress, pollution, maternal age and education, and to the quality of the nearest hospital.

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Go deeper

• The report: every estimate with its interval and sample size.
• Tables: every figure of the report as a table.

Reading paths that include this chapter:

• The argument in eight chapters
• Disadvantage as a dose
• Race and place

Data sources

• CDC WONDER natalityCDC WONDER natality, county by birthweight and gestational age, births in 2018 to 2024Years: 2018-2024 · Weights: none (full counts of births); county shares weighted by births · License: Public domain (NCHS, CDC); WONDER data-use restrictions apply
• Area Deprivation IndexNeighborhood Atlas Area Deprivation Index 2023 (v4.0.1), tract national rank, averaged to counties with 2019-2023 tract populationYears: 2023 release (version 4.0.1; built on ACS 2019-2023) · Weights: 2019-2023 tract population · License: Free for non-commercial use; cite the Neighborhood Atlas and Kind and Buckingham
• County Health RankingsCounty Health Rankings 2024, low birthweight (births in 2016 to 2022), overall and for non-Hispanic Black, non-Hispanic white and Hispanic mothersYears: 2024 release · Weights: births (recovered from published intervals for race groups) · License: CC BY 4.0 (University of Wisconsin Population Health Institute)
• CDC WONDER linked birth and infant deathCDC WONDER linked birth and infant death, mother's single race (any ethnicity), 2018 and 2019Years: 2018-2022 · Weights: none (crude rates from counts) · License: Public domain (NCHS, CDC); WONDER data-use restrictions apply

All data sources

References

1. Kind, Amy J. H.; Buckingham, William R. (2018). Making Neighborhood-Disadvantage Metrics Accessible: The Neighborhood Atlas. New England Journal of Medicine 378(26): 2456-2458. doi:10.1056/NEJMp1802313
2. Geronimus, Arline T. (1992). The Weathering Hypothesis and the Health of African-American Women and Infants: Evidence and Speculations. Ethnicity & Disease 2(3): 207-221.
3. Hoynes, Hilary; Miller, Doug; Simon, David (2015). Income, the Earned Income Tax Credit, and Infant Health. American Economic Journal: Economic Policy 7(1): 172-211. doi:10.1257/pol.20120179

How to cite

@misc{singh2026birthweightandpretermbirth,
  author = {Singh, Vishal},
  title = {Birthweight and preterm birth},
  year = {2026},
  url = {https://vishalsingh.org/zipcode/birthweight-and-preterm-birth},
  note = {In ZIP Code Destiny. Published 9 October 2026.}
}