Key Points
Question
What is the association between maternal birthplace and preterm birth rates, and are there differences by self-reported race and ethnicity of pregnant individuals?
Findings
In this cross-sectional study of 8 590 988 nulliparous individuals with a live birth in the US between 2014 and 2019, preterm birth rates were significantly lower among non–US-born individuals compared with US-born individuals. Heterogeneity was observed within racial and ethnic groups, and non-Hispanic Black individuals had the largest relative difference between non–US-born and US-born individuals.
Meaning
These data support existing evidence that preterm birth rates are lower among individuals born outside of the US compared to those born in the US and underscore the heterogeneity across disaggregated racial and ethnic subgroups.
This cross-sectional study evaluates associations between preterm birth rates and place of birth as well as race and ethnicity.
Abstract
Importance
Preterm birth is a major contributor to neonatal morbidity and mortality, and considerable differences exist in rates of preterm birth among maternal racial and ethnic groups. Emerging evidence suggests pregnant individuals born outside the US have fewer obstetric complications than those born in the US, but the intersection of maternal nativity with race and ethnicity for preterm birth is not well studied.
Objective
To determine if there is an association between maternal nativity and preterm birth rates among nulliparous individuals, and whether that association differs by self-reported race and ethnicity of the pregnant individual.
Design, Setting, and Participants
This was a nationwide, cross-sectional study conducted using National Center for Health Statistics birth registration records for 8 590 988 nulliparous individuals aged 15 to 44 years with singleton live births in the US from 2014 to 2019. Data were analyzed from March to May 2022.
Exposures
Maternal nativity (non–US-born compared with US-born individuals as the reference, wherein US-born was defined as born within 1 of the 50 US states or Washington, DC) in the overall sample and stratified by self-reported ethnicity and race, including non-Hispanic Asian and disaggregated Asian subgroups (Asian Indian, Chinese, Filipino, Japanese, Korean, Pacific Islander, Vietnamese, and other Asian), non-Hispanic Black, Hispanic and disaggregated Hispanic subgroups (Cuban, Mexican, Puerto Rican, and other Hispanic), and non-Hispanic White.
Main Outcomes and Measures
The primary outcome was preterm birth (<37 weeks of gestation) and the secondary outcome was very preterm birth (<32 weeks of gestation).
Results
Of 8 590 988 pregnant individuals included (mean [SD] age at delivery, 28.3 [5.8] years in non–US-born individuals and 26.2 [5.7] years in US-born individuals; 159 497 [2.3%] US-born and 552 938 [31.2%] non–US-born individuals self-identified as Asian or Pacific Islander, 1 050 367 [15.4%] US-born and 178 898 [10.1%] non–US-born individuals were non-Hispanic Black, 1 100 337 [16.1%] US-born and 711 699 [40.2%] non–US-born individuals were of Hispanic origin, and 4 512 294 [66.1%] US-born and 328 205 [18.5%] non–US-born individuals were non-Hispanic White), age-standardized rates of preterm birth were lower among non–US-born individuals compared with US-born individuals (10.2%; 95% CI, 10.2-10.3 vs 10.9%; 95% CI, 10.9-11.0) with an adjusted odds ratio (aOR) of 0.90 (95% CI, 0.89-0.90). The greatest relative difference was observed among Japanese individuals (aOR, 0.69; 95% CI, 0.60-0.79) and non-Hispanic Black individuals (aOR, 0.74; 0.73-0.76) individuals. Non–US-born Pacific Islander individuals experienced higher preterm birth rates compared with US-born Pacific Islander individuals (aOR, 1.15; 95% CI, 1.04-1.27). Puerto Rican individuals born in Puerto Rico compared with those born in US states or Washington, DC, also had higher preterm birth rates (aOR, 1.07; 95% CI, 1.03-1.12).
Conclusions and Relevance
Overall preterm birth rates were lower among non–US-born individuals compared with US-born individuals. However, there was substantial heterogeneity in preterm birth rates across maternal racial and ethnic groups, particularly among disaggregated Asian and Hispanic subgroups.
Introduction
Individuals born prematurely are at higher risk of adverse health outcomes in both the short term during the neonatal period as well as in the long term into adulthood.1,2,3,4 Rates of preterm birth have increased in the US in recent years, with an annual increase of 2% per year from 2014 to 2019, and considerable racial and ethnic disparities exist with the highest rate of preterm birth among non-Hispanic Black individuals.5 A variety of maternal factors, including socioeconomic status, access to health care, implicit bias, perceived discrimination, and environmental exposures have each been associated with risk of preterm birth.6,7,8 These upstream factors may partially account for the disproportionate burden of preterm birth experienced by racial and ethnic minoritized groups in the US.9,10 In addition, rates of preterm birth are considerably higher in the US compared with other high-income countries.11 However, little is known regarding differences in risk of preterm birth at the intersection of maternal nativity (birthplace of the pregnant person themselves) and maternal racial and ethnic identity.
Prior work has demonstrated that rates of hypertensive disorders of pregnancy, a leading contributor to preterm birth, are lower among individuals born outside the US compared with those born inside the US.12,13,14 However, the opposite pattern was observed for gestational diabetes; individuals born outside the US had higher rates of gestational diabetes than those born inside the US.12 These findings suggest the intersection of maternal nativity with adverse pregnancy outcomes are complex and not consistent with an oversimplified concept of the “healthy immigrant effect.”12,13 Therefore, this study aims to quantify contemporary rates and relative differences in preterm birth among individuals born outside the US compared with US-born individuals by self-identified race and ethnic groups and disaggregated subgroups between 2014 and 2019.
Methods
We used data from the National Center for Health Statistics (NCHS) birth registration records to calculate rates of preterm birth in the US. The study period spanned 2014 to 2019 when maternal nativity was made publicly available and before the onset of the COVID-19 pandemic. NCHS records include all live births in the US. Data are collected by the health care professional attendant at birth and are based on maternal self-report, prenatal records, and the labor and delivery admission based on a standard protocol by NCHS. In this analysis, we included pregnant individuals with a live birth aged 15 to 44 years at delivery. Records from individuals with missing data on gestational age at delivery, maternal nativity, and maternal race or ethnicity information were excluded. We did not evaluate individuals who identified as American Indian or Alaska Native since all members of this group were born in the US. We excluded multiparous individuals to avoid double counting those who gave birth multiple times during the study period. Additionally, prior preterm birth is a known risk factor for subsequent preterm birth.15 Multiple births (eg, twins and triplets) were also excluded due to the differences in risk of preterm birth for such pregnancies.16 The final analytic sample consisted of 8 590 988 live births (Figure 1). The institutional review board at Northwestern University deemed the study to be exempt because of the deidentified, publicly available nature of the data set. The study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline. Data were analyzed from March to May 2022.
Figure 1. Analytic From All Birthing Individuals Aged 15 to 44 Years in the US With a First Singleton Live Birth, 2014-2019.
aIn the regression analysis, the sample size was reduced to 8 044 521 individuals due to missing data on the covariates.
Study Measures
The primary outcome of interest was preterm birth, defined as birth before 37 weeks of gestation. A secondary outcome of very preterm birth (before 32 weeks of gestation) was also included given the clinical relevance of this perinatal outcome and its association with higher risk of neonatal morbidity and mortality as defined in prior publications.17,18 Nativity was defined based on maternal self-report of place of birth outside the US or within the US (defined as born in 1 of the 50 US states or Washington, DC). The social constructs of race and ethnicity were self-reported, and included the following categories from the US Standard Certificate of Live Birth: non-Hispanic Asian overall and Asian subgroups (Asian Indian, Chinese, Filipino, Japanese, Korean, Pacific Islander, Vietnamese, and other Asian), non-Hispanic Black, Hispanic overall and Hispanic subgroups (Cuban, Mexican, Puerto Rican, Cuban, and other Hispanic), and non-Hispanic White. We stratified first by self-identified ethnicity (Hispanic: yes or no) followed by self-identified racial groups and subgroups. Covariates included the birthing individual’s age, highest level of education, insurance type at delivery, prepregnancy body mass index (BMI), and prepregnancy hypertension and prepregnancy diabetes status.
Statistical Analysis
Age-standardized rates for each outcome (preterm and very preterm) were calculated per 1000 live births overall (in all race and ethnicity groups) and stratified first by primary race and ethnic groups and second by Asian and Hispanic subgroups. Rates were age-standardized to the age distribution of individuals who gave birth in 2014 (the first year of our sample data). We performed logistic regression analyses to calculate adjusted odds ratios (aORs) and 95% CIs to quantify the associations between maternal nativity status and preterm birth for each self-reported maternal race and ethnic group and subgroup for births between 2014 and 2019. We used US-born individuals as the reference group in the analyses as this was the majority population in birthing adults. Adjustment covariates included fixed effects for delivery year and sociodemographic variables (birthing individual’s age, education attainment [less than high school, high school graduate, some college, or college graduate and above], insurance at delivery [Medicaid, private insurance, self-pay, or other sources]), and prepregnancy health factors, including prepregnancy BMI (underweight, normal, overweight, class 1 obesity, class 2 obesity, or class 3 obesity), prepregnancy hypertension, and prepregnancy diabetes. We first performed the regression models for the full sample and then for each race and ethnic group. To calculate the sensitivity to unmeasured confounding, we calculated E-values. All regression analyses were performed in STATA version 17 (StataCorp).
In an additional analysis, preterm birth rates were calculated per year between 2014 and 2019. We used the Joinpoint Regression statistical tool version 4.8.0.1 (National Cancer Institute) to calculate the average annual percent change of age-standardized rates of preterm birth in each year and among each racial and ethnic group and subgroup. For all analyses, 2-sided P < .05 indicated statistical significance.
Results
Among 8 590 988 nulliparous individuals with a singleton live birth between 2014 and 2019, 1 771 740 individuals born outside the US and 6 822 495 individuals born in the US were included in the analysis. Among those born outside the US, the mean (SD) age at delivery was 28.3 (5.8) years compared with 26.2 (5.7) years for US born individuals (Table 1). Of those born outside the US, 552 938 (31.2%) were Asian and Pacific Islander, 178 898 (10.1%) were non-Hispanic Black, 711 699 (40.2%) were Hispanic, and 328 205 (18.5%) were non-Hispanic White. Of those who were born in the US, 159 497 (2.3%) were Asian and Pacific Islander, 1 050 367 (15.4%) were non-Hispanic Black; 1 100 337 (16.1%) were Hispanic; and 4 512 294 (66.1%) were non-Hispanic White. Compared with those born in the US, individuals born outside the US were less likely to have a high school degree (1 466 303 [85.7%] vs 6 064 545 [90.5%]) or have private insurance at delivery (816 943 [47.1%] vs 3 873 343 [57.6%]). Individuals born outside the US had lower median (IQR) prepregnancy BMI (23.4 [20.0-25.9] vs 24.9 [20.8-29.0], calculated as weight in kilograms divided by height in meters squared) and lower frequency of prepregnancy hypertension (16 144 [0.9%] vs 121 791 [1.8%]) and diabetes (11 689 [0.7%] vs 53 323 [0.8%]) compared with those born in the US.
Table 1. Characteristics of Birthing Individuals by Nativity Status in the US, 2014-2019a.
| Characteristic | No. (%) | ||
|---|---|---|---|
| Born in the US (n = 6 887 819)b | Born outside the US (n = 1 772 771) | Standardized mean difference (95% CI) | |
| Age, mean (SD), y | 26.2 (5.7) | 28.3 (5.8) | −0.36 (−0.37 to −0.35) |
| Race and ethnicity | |||
| All Asian and Pacific Islander | 159 497 (2.3) | 552 938 (31.2) | −0.84 (−0.86 to −0.82) |
| Asian Indian | 46 684 (0.7) | 192 366 (10.9) | −0.45 (−0.45 to −0.45) |
| Chinese | 24 123 (0.4) | 144 293 (8.1) | −0.39 (−0.39 to −0.39) |
| Filipino | 22 092 (0.3) | 52 017 (2.9) | −0.21 (−0.22 to −0.20) |
| Japanese | 5104 (0.1) | 12 733 (0.7) | −0.10 (−0.11 to −0.09) |
| Korean | 10 113 (0.1) | 31 139 (1.8) | −0.17 (−0.17 to −0.17) |
| Pacific Islander | 7395 (0.1) | 10 101 (0.6) | −0.08 (−0.08 to −0.08) |
| Vietnamese | 12 412 (0.2) | 39 936 (2.3) | −0.19 (−0.19 to −0.19) |
| Other Asian | 31 574 (0.5) | 70 353 (4) | −0.24 (−0.25 to −0.23) |
| Non-Hispanic Black | 1 050 367 (15.4) | 178 898 (10.1) | 0.16 (−0.17 to −0.15) |
| All Hispanic | 1 100 337 (16.1) | 711 699 (40.2) | −0.55 (−0.56 to −0.54) |
| Cuban | 26 270 (0.4) | 35 090 (2) | −0.15 (−0.15 to −0.15) |
| Mexican | 658 445 (9.7) | 330 008 (18.6) | −0.26 (−0.26 to −0.26) |
| Puerto Rican | 120 640 (1.8) | 35 168 (2) | −0.02 (−0.02 to −0.02) |
| Other Hispanic | 294 982 (4.3) | 311 433 (17.6) | −0.43 (−0.44 to −0.42) |
| Non-Hispanic White | 4 512 294 (66.1) | 328 205 (18.5) | 1.10 (1.10 to 1.10) |
| Education | |||
| <High school | 637 721 (9.5) | 244 421 (14.3) | −0.15 (−0.15 to −0.15) |
| High school graduate | 1 661 426 (24.8) | 364 348 (21.3) | 0.08 (0.07 to 0.09) |
| Some college | 2 025 033 (30.2) | 353 056 (20.6) | 0.22 (0.22 to 0.22) |
| ≥College graduate | 2 378 086 (35.5) | 748 899 (43.8) | −0.17 (−0.17 to −0.17) |
| Delivery insurance | |||
| Medicaid | 2 473 125 (36.8) | 678 940 (39.1) | −0.05 (−0.06 to −0.04) |
| Private insurance | 3 873 343 (57.6) | 816 943 (47.1) | 0.21 (0.20 to 0.22) |
| Self-pay | 117 485 (1.7) | 168 532 (9.7) | −0.35 (−0.35 to −0.35) |
| Other source | 258 404 (3.8) | 70 575 (4.1) | −0.01 (−0.01 to −0.01) |
| Prepregnancy body mass index, median (IQR)c | 24.9 (20.8-29.0) | 23.4 (20.0-25.9) | 0.37 (0.37 to 0.37) |
| Prepregnancy hypertension | 121 791 (1.8) | 16 144 (0.9) | 0.08 (0.07 to 0.09) |
| Prepregnancy diabetes | 53 323 (0.8) | 11 689 (0.7) | 0.01 (0.01 to 0.01) |
| Preterm birth | |||
| All preterm birth (<37 wk gestation) | 664 077 (9.7) | 158 600 (9) | 0.03 (0.03 to 0.03) |
| Very preterm birth (<32 w1k gestation) | 120 266 (1.8) | 26 293 (1.5) | 0.02 (0.02 to 0.02) |
The sample includes individuals aged 15 to 44 years with a first singleton birth from 2014 to 2019.
Born in the US is defined as born in 1 of the 50 US states or Washington, DC.
Calculated as weight in kilograms divided by height in meters squared.
Overall, the age-standardized rate of preterm birth was 10.2% (95% CI, 10.2-10.3) in those born outside the US and 10.9% (95% CI, 10.9-11.0) in those born in the US (eTable 1 in Supplement 1). Age-standardized rates of preterm birth varied across race and ethnicity groups and subgroups, with highest rates among non-Hispanic Black individuals (born outside the US: 13.3%; 95% CI, 13.0-13.5 and US-born: 16.8%; 95% CI, 16.6-16.9). There was heterogeneity in preterm birth rates among Asian and Pacific Islander subgroups and Hispanic subgroups, with the highest rates of preterm birth among Pacific Islander individuals (born outside the US: 16.3%; 95% CI, 15.0-17.7 and US-born: 14.9%; 95% CI, 13.2-16.6) and Puerto Rican individuals (born outside the US states or Washington, DC: 13.2%; 95% CI, 12.5-13.8 and within the US states or Washington, DC: 12.3%; 11.9-12.7), respectively.
The crude preterm birth rates by race, ethnicity, and nativity status are shown in Table 2. The ORs, aORs, and 95% CIs for preterm birth among race and ethnic groups and subgroups are shown in Figure 2 and Table 3. Overall risk of preterm birth was lower for individuals born outside the US than those born inside the US (aOR, 0.90; 95% CI, 0.89-0.90) adjusted for sociodemographic and prepregnancy covariates. Among the 4 primary race and ethnic groups, patterns were similar (Asian and Pacific Islander: aOR, 0.91; 95% CI, 0.89-0.92, non-Hispanic Black: aOR, 0.74; 95% CI, 0.73-0.76, Hispanic: aOR, 0.95; 95% CI, 0.94-0.97, and non-Hispanic White: aOR, 0.84; 95% CI, 0.83-0.86). Among non-Hispanic Asian or Pacific Islander subgroups, there was heterogeneity with lower risk of preterm birth for non–US-born compared with US-born individuals among Asian Indian (aOR, 0.93; 95% CI, 0.90-0.97), Chinese (aOR, 0.78; 95% CI, 0.73-0.83), Japanese (aOR, 0.69; 95% CI, 0.60-0.79), and Vietnamese (aOR, 0.87; 95% CI, 0.81-0.94) subgroups. In contrast, there was higher risk of preterm birth for non–US-born compared with US-born individuals among Filipino (aOR, 1.05; 95% CI, 0.99-1.11) and Pacific Islander (aOR, 1.15; 95% CI, 1.04-1.27) subgroups and no significant difference among Korean individuals born outside the US compared with those born in the US. Among Hispanic subgroups, different patterns were observed with lower preterm birth rates among Mexican individuals born outside vs in the US (aOR, 0.98; 95% CI, 0.97-0.99), higher preterm birth rates among Puerto Rican individuals born outside vs within US states or Washington, DC (aOR, 1.09; 95% CI, 1.05-1.14), and no significant difference among Cuban individuals. E-values are shown in Table 3 for all of the models. Among all models with statistically significant results, the E-values for aORs are all greater than 1 and range from 1.21 to 2.28.
Table 2. Preterm (<37 Weeks’ Gestation) Birth Rates of Birthing Individuals Among Self-Reported Race and Ethnicity Groups by Nativity Status in the US, 2014-2019a.
| Race and ethnicity | Nativityb | Crude rates, % |
|---|---|---|
| Overall | US-born | 9.7 |
| Non–US-born | 9 | |
| All Asian and Pacific Islander | US-born | 9.4 |
| Non–US-born | 8 | |
| Asian Indian | US-born | 9.4 |
| Non–US-born | 8.2 | |
| Chinese | US-born | 7.6 |
| Non–US-born | 5.2 | |
| Filipino | US-born | 11 |
| Non–US-born | 11.4 | |
| Japanese | US-born | 9 |
| Non–US-born | 6.2 | |
| Korean | US-born | 6.5 |
| Non–US-born | 6.4 | |
| Pacific Islander | US-born | 12.7 |
| Non–US-born | 14.8 | |
| Vietnamese | US-born | 9.5 |
| Non–US-born | 9 | |
| Other Asian | US-born | 9.8 |
| Non–US-born | 10 | |
| Non-Hispanic Black | US-born | 14.3 |
| Non–US-born | 11.5 | |
| All Hispanic | US-born | 9.7 |
| Non–US-born | 9.8 | |
| Cuban | US-born | 10.4 |
| Non–US-born | 10.6 | |
| Mexican | US-born | 9.4 |
| Non–US-born | 9.3 | |
| Puerto Rican | US-born | 10.6 |
| Non–US-born | 11.5 | |
| Other Hispanic | US-born | 9.9 |
| Non–US-born | 10.2 | |
| Non-Hispanic White | US-born | 8.7 |
| Non–US-born | 7.2 |
The sample includes individuals aged 15 to 44 years with a first singleton birth from 2014 to 2019. Age-standardized rates are based on the age distribution of the 2014 data.
US-born is defined as born in 1 of the 50 US states or Washington, DC.
Figure 2. Associations Between Maternal Nativity Status and Preterm Birth Rates Among Self-Reported Race and Ethnicity Groups of Birthing Individuals Aged 15 to 44 Years in the US, 2014-2019.
Each plot is based on a unique regression model showing the adjusted odds ratios and 95% CIs of the nativity variable for each racial and ethnic group. All models were adjusted for age, education attainment, insurance at delivery, prepregnancy body mass index, prepregnancy hypertension, prepregnancy diabetes, and year fixed effects. An adjusted odds ratio greater than 1 suggests that individuals born outside the US had greater odds of having preterm birth than those born in the US. US born is defined as born in 1 of the 50 US states or Washington, DC.
Table 3. Associations Between Nativity Status of the Birthing Individual and Preterm Birth Rates Among Self-Reported Race and Ethnicity Groups and Disaggregated Asian and Hispanic Subgroups in the US, 2014-2019.
| Race and ethnicity | Adjusted odds ratio (95% CI)a | E-value | |
|---|---|---|---|
| Model 1b | Model 2c | ||
| Overall | 0.91 (0.91-0.92)d | 0.90 (0.89-0.90)d | 1.48 |
| All Asian and Pacific Islander | 0.84 (0.82-0.85)d | 0.91 (0.89-0.92)d | 1.44 |
| Asian Indian | 0.86 (0.83-0.89)d | 0.93 (0.90-0.97)d | 1.35 |
| Chinese | 0.66 (0.63-0.70)d | 0.78 (0.73-0.83)d | 1.89 |
| Filipino | 1.04 (0.99-1.09) | 1.05 (0.99-1.11) | 1.28 |
| Japanese | 0.67 (0.60-0.76)d | 0.69 (0.60-0.79)d | 2.28 |
| Korean | 0.99 (0.90-1.08) | 1.01 (0.91-1.11) | 1.10 |
| Pacific Islander | 1.20 (1.10-1.31)d | 1.15 (1.04-1.27)d | 1.56 |
| Vietnamese | 0.94 (0.87-1.00) | 0.87 (0.81-0.94)d | 1.55 |
| Other Asian | 1.03 (0.98-1.07) | 1.01 (0.96-1.06) | 1.11 |
| Non-Hispanic Black | 0.78 (0.77-0.79)d | 0.74 (0.73-0.76)d | 2.03 |
| All Hispanic | 1.01 (1.00-1.02)d | 0.95 (0.94-0.97)d | 1.27 |
| Cuban | 1.02 (0.97-1.07) | 1.00 (0.94-1.06) | 1.05 |
| Mexican | 0.98 (0.97-0.99)d | 0.91 (0.89-0.92)d | 1.43 |
| Puerto Rican | 1.09 (1.05-1.14)d | 1.07 (1.03-1.12)d | 1.36 |
| Other Hispanic | 1.03 (1.01-1.05)d | 0.97 (0.95-0.99)d | 1.21 |
| Non-Hispanic White | 0.82 (0.81-0.83)d | 0.84 (0.83-0.86)d | 1.65 |
Each cell is based on a different regression model showing the adjusted odds ratio and 95% confidence intervals of the nativity variable for each racial and ethnic group indicated by the row headings.
An adjusted odds ratio greater than 1 suggests that individuals born outside the US had greater odds of having preterm birth than those born in the US. Born in the US is defined as born in 1 of the 50 US states or Washington, DC. 95% CIs are based on robust standard errors.
Model 1 is unadjusted.
Model 2 is adjusted for age, education attainment, insurance type at delivery, prepregnancy body mass index, prepregnancy hypertension, prepregnancy diabetes, and year fixed effects.
The result is statistically significant at the .05 level.
In secondary analyses, similar patterns were observed for very preterm birth rates with lower rates among individuals born outside the US than those born in the US (eTable 2 in Supplement 1). In adjusted models, non-Hispanic Black individuals had the greatest relative difference with 34% lower risk of very preterm birth among those born outside the US compared with those born in the US (aOR, 0.66; 95% CI, 0.64-0.69) (eTable 3 in Supplement 1). Similar heterogeneity was observed among Asian and Hispanic subgroups for very preterm and preterm birth rates. In trends analysis for annual age-standardized rates of preterm birth between 2014 and 2019, the overall average annual percent change demonstrated increases in both individuals born outside the US (2.2% per year [1.5-3.0]) and those born in the US (1.5% per year [1.1-2.0]) (eTable 4 in Supplement 1). While significant increases also occurred in individuals born outside the US for very preterm births (average annual percent change, 2.4 [1.1-3.6]), there were no significant increases in US born individuals.
Discussion
In this national cross-sectional analysis of all nulliparous individuals in the US with a singleton live birth between 2014 and 2019, rates of preterm birth and very preterm birth were significantly lower among pregnant individuals who themselves were born outside the US compared with US-born individuals. The preterm birth rate was the highest among non-Hispanic Black individuals born in the US. The greatest relative difference in preterm birth among non–US-born and US-born individuals was also observed for non-Hispanic Black individuals among all summary racial and ethnic groups. Rates of preterm birth increased in most groups during the study period.
These findings extend previous research regarding the importance of nativity or maternal birthplace as an independent and important social determinant of health to consider. Nativity may represent a surrogate or proxy of a variety of unique challenges and opportunities experienced as a result of immigration that are associated with health outcomes. A similar association was recently reported in a smaller cohort sample from the Boston Birth Cohort among 6096 birthing individuals with a lower risk of preterm birth observed among those born outside the US with less than 10 years of residing in the US but only included non-Hispanic Black individuals.13,19 The current study demonstrated a higher risk of preterm birth among certain Asian and Hispanic subgroups (Filipino, Pacific Islander, and Puerto Rican) when the birthing individual is born outside the US, which may reflect the larger proportion of refugees and workers in agriculture and domestic service industries among these immigrant groups with greater social adversity experienced and associated challenges in accessing and interacting with health care than those from other countries of origin.20 Although Puerto Rican individuals born in Puerto Rico are US citizens and face fewer migration barriers than immigrant groups, prior studies have demonstrated that Puerto Rico–born Puerto Rican individuals may face many of the same challenges as other immigrant groups due to linguistic, cultural, and socioeconomic differences between Puerto Rico and US states.21 Factors contributing to Puerto Rican–born Puerto Rican individuals having worse birth outcomes in the US may include the relatively low levels of education and higher poverty rate compared with other Hispanic immigrant groups.22 Puerto Rican–born Puerto Rican individuals may also be more likely to live in highly segregated areas, which could also contribute to the disparities in preterm birth observed in this analysis.21
The greatest relative difference among non-Hispanic Black individuals born outside the US compared with those born in the US is consistent with patterns observed in prior national studies for hypertensive disorders of pregnancy.23 This may be related to experiences of structural and interpersonal racism that disproportionately affect non-Hispanic Black individuals living in the US.24 Potential mechanisms that may mediate these findings include perceived stress and weathering whereby repeated experiences of racism can magnify the association between an adverse health factor and outcome.25,26 In a recent analysis from the Boston Birth Cohort sample, longer duration of residence in the US was associated with higher risk of preeclampsia, which may in part be related to greater exposure to discrimination in addition to acculturation with adoption and changes in lifestyle behaviors and dietary patterns.14 All of these adverse exposures are associated with higher risk of adverse pregnancy outcomes that have important implications for the long-term health of the offspring.27 Importantly, we included individuals who identified as Hispanic Black within the Hispanic group. Approximately 8.7% of Black birthing individuals self-identified as Hispanic. Among Hispanic Black individuals, about 38.5% were non–US-born, which closely aligns with the 39.1% non-US-born rate among all Hispanic populations (eTable 5 in Supplement 1). In contrast, 14% of non-Hispanic Black individuals in our sample were non–US-born. This suggests that the nativity patterns of individuals who identified as Hispanic Black were similar to those of the overall Hispanic population.
Although Hispanic and Asian individuals not born in the US had lower risk of preterm birth compared with their US-born counterparts, it is worth noting that issues that may influence health, such as xenophobia, bias, discrimination, and colorism, may influence both non–US- and US-born groups.28 An individual’s place of birth and time lived in the host country are closely related to various underlying factors that influence their perception of discrimination. These factors include immigration-related policies, language barriers, and the sense of being and outsider or treated as such.29,30,31 These factors could also directly affect birthing individuals’ prenatal stress, diet quality, and social support during pregnancy.32,33 While the reasons for better birth outcomes among non–US-born Hispanic and Asian individuals may be multifactorial, the potential for acculturation in US-born Hispanic and Asian individuals, leading to adoption of unhealthy behaviors may play a role but remains understudied in these pregnant groups. This is an important and understudied area that warrants further investigation. Significant differences were observed among disaggregated Asian and Hispanic subgroups and underscores that US racial and ethnic as well as immigrant populations are heterogeneous. Such differences may again, in part, be related to immigration experiences, years lived in the US, and acculturation.34,35,36
The complex intersection between nativity with race and ethnicity has also been documented for a variety of other health outcomes, such as hypertension and diabetes in nonpregnant populations.37,38,39 Given the biologic processes leading to risk of preterm birth begin long before conception, which are also associated with prepregnancy hypertension and diabetes, it is important to consider a comprehensive array of social determinants of health before and during pregnancy in preventive interventions.40,41 While preterm birth is a well-established factor for adverse offspring outcomes in the short-term during the neonatal period as well as in the long-term through adulthood, emerging evidence supports preterm birth as a risk factor for lifetime risk of cardiovascular disease among the birthing individual.42
From 2014 to 2019, most groups experienced increases in age-standardized preterm birth rates. After years of decline in preterm birth since 2007, these documented national-level increases may be related to increasing rates of other adverse pregnancy outcome (eg, hypertensive disorders of pregnancy and gestational diabetes) with approximately 1 in 5 births now complicated by an adverse pregnancy outcome in the US.43 While there were similar increases in preterm birth rates for individuals born outside and in the US, there was divergence in patterns with statistically significant increases in very preterm only among those born outside the US.
Limitations
There are several limitations to this study. First, despite efforts to account for confounding through adjusting for observed confounders, the potential influence of unmeasured confounders may persist. Therefore, we computed E-values, which confirmed relatively minor influence of residual confounding.44 Second, nativity or birthplace of the pregnant individual is dichotomized and covariates regarding age at immigration, years lived in the US, and immigration experience were not available. Therefore, the nativity variable may not fully represent the cultural background and experiences of the birthing persons, such as duration of residence in the US and the age of immigration that are also associated with health outcomes.19 Third, while we included a comparison of Puerto Rican individuals born outside US states with those born within US states, we could not specifically distinguish who was born in Puerto Rico or born outside of the commonwealth or US states. Fourth, birth registration data may be subject to miscoding. However, this record is completed by a professional birth attendant, and the gestational age at delivery have been validated with hospital records with a high degree of concordance.
Conclusions
In this study, preterm birth rates differed significantly by maternal nativity in the US with lower rates among pregnant individuals born outside the US compared with those born in the US individuals among most race and ethnicity groups. However, there was considerable heterogeneity, particularly among Asian and Hispanic disaggregated subgroups that required detailed analysis beyond the larger Asian and Hispanic categories. The intersection of nativity with race and ethnicity in the assessment of social determinants of health should be incorporated when considering interventions to improve maternal health and neonatal outcomes. In addition, given the lifelong and intergenerational outcomes following preterm birth, understanding mechanisms between the associations between maternal nativity status, acculturation, and preterm birth rates will be critical.
eTable 1. Preterm birth rates (per 100 live births) by nativity status of birthing individuals among self-reported race and ethnicity groups and disaggregated Asian and Hispanic subgroups in the US, 2014-2019
eTable 2. Very preterm birth rates (per 100 live births) by nativity status of birthing individuals among self-reported race and ethnicity groups and disaggregated Asian and Hispanic subgroups in the US, 2014-2019
eTable 3. Associations between nativity status of the birthing individual and very preterm birth (defined as less than 32 weeks’ gestation) among self-reported race and ethnicity groups and disaggregated Asian and Hispanic subgroups in the US, 2014-2019
eTable 4. Average annual percent changes in preterm birth rates by maternal nativity status among self-reported race and ethnicity groups and disaggregated Asian and Hispanic subgroups in the US, 2014-2019
eTable 5. Nativity patterns among birthing individuals who identified as Hispanic White, Black, and Asian Pacific Islander in the US, 2014-2019
Data sharing statement
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
eTable 1. Preterm birth rates (per 100 live births) by nativity status of birthing individuals among self-reported race and ethnicity groups and disaggregated Asian and Hispanic subgroups in the US, 2014-2019
eTable 2. Very preterm birth rates (per 100 live births) by nativity status of birthing individuals among self-reported race and ethnicity groups and disaggregated Asian and Hispanic subgroups in the US, 2014-2019
eTable 3. Associations between nativity status of the birthing individual and very preterm birth (defined as less than 32 weeks’ gestation) among self-reported race and ethnicity groups and disaggregated Asian and Hispanic subgroups in the US, 2014-2019
eTable 4. Average annual percent changes in preterm birth rates by maternal nativity status among self-reported race and ethnicity groups and disaggregated Asian and Hispanic subgroups in the US, 2014-2019
eTable 5. Nativity patterns among birthing individuals who identified as Hispanic White, Black, and Asian Pacific Islander in the US, 2014-2019
Data sharing statement