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. Author manuscript; available in PMC: 2020 Sep 1.
Published in final edited form as: Paediatr Perinat Epidemiol. 2019 Jul 31;33(5):346–356. doi: 10.1111/ppe.12572

Risks of preterm birth among non-Hispanic black and non-Hispanic white women: Effect modification by maternal age

Amaris M Keiser 1, Yasmmyn D Salinas 2, Andrew T DeWan 2, Nicola L Hawley 2, Pamela K Donohue 1,3, Donna M Strobino 3
PMCID: PMC6993282  NIHMSID: NIHMS1040952  PMID: 31365156

Abstract

Background:

Preterm birth (PTB) disproportionately affects African-American compared to Caucasian women, although reasons for this disparity remain unclear. Some suggest that a differential effect of maternal age by race/ethnicity, especially at older maternal ages, may explain disparities.

Objective:

To determine whether the relationship between maternal age and preterm birth varies by race/ethnicity among primiparae non-Hispanic blacks (NHB) and non-Hispanic whites (NHW).

Methods:

A cross-sectional study of 367,081 singleton live-born first births to NHB and NHW women in California from 2008–2012 was conducted. Rate ratios (RR) were estimated for PTB and its subtypes – spontaneous and clinician-initiated – after adjusting for confounders through Poisson regression. Universal age/race reference groups (NHW, 25–29 years) and race-specific reference groups (NHW or NHB, 25–29 years) were used for comparisons.

Results:

Among all women, RR of PTB was highest at the extremes of age (<15 and ≥40 years). Among NHBs, the risk of PTB was higher than among NHWs at all maternal ages (adjusted RR of PTB 1.38 to 2.93 versus 0.98 to 2.38). However, using race-specific reference groups, the risk of PTB for NHB women (RR 0.91 to 1.88) versus NHW women (RR 0.98 to 2.39) was nearly identical at all maternal ages, with overlapping confidence intervals. Analyses did not demonstrate substantial divergence of risk with advancing maternal age. PTB, spontaneous PTB and clinician-initiated PTB demonstrated similar risk patterns at younger but not older maternal ages, where risk of clinician-initiated PTB increased sharply for all women.

Conclusions:

Primiparae NHBs demonstrated increased risk of PTB, spontaneous and clinician-initiated PTB compared to NHWs at all maternal ages. However, RRs using race-specific reference groups converged across maternal ages, indicating a similar independent effect of maternal age on PTB by race/ethnicity. A differential effect of maternal age does not appear to explain disparities in preterm birth by race/ethnicity.

Keywords: Black, disparities, maternal age, preterm birth, weathering

Background

In the United States, non-Hispanic black women experience higher rates of infant mortality, preterm birth (<37 weeks) and small for gestational age births compared to non-Hispanic white women, even after adjustment for socioeconomic, health and behavioral factors.1,2 Though exact mechanisms remain unclear, one proposed hypothesis, known as “weathering”, attributes poorer birth outcomes among black women to earlier and more rapid deterioration of health with advancing age.3 This earlier and steeper health decline is conceptualised as a direct consequence of lifelong exposure to social and environmental disadvantage, which ultimately leads to widening health disparities between blacks and whites as they age. While initially presented as an explanation for excess rates of infant mortality among black women,3,4 it is now applied to explain broader disparities in health outcomes among disadvantaged populations.5–8

Additional hypotheses, including normative differences in childbearing patterns by age,9 allostatic load,6,10–12 the developmental origins of health and disease,13 and the life-course framework 11,14–16 have been developed to advance our understanding of disparities in birth outcomes.16–24 The extent to which the various hypotheses apply to all minority women at all points in their reproductive lives is not clear. For weathering specifically, researchers often use white women in their twenties as the universal reference group, an approach that does not sufficiently account for the increased baseline risk of poor outcomes among black women at all ages, nor for the increasing risk of poor birth outcomes with advancing maternal age for all women.25–28 Furthermore, age is frequently grouped into broad, uneven categories and fails to include women at the tails of the distribution (below 20 years or over 35 years), a particular shortcoming as childbearing patterns have shifted toward later initiation of births among US women.29,30 In addition, many early studies included only limited adjustment for multiple confounders, especially maternal health conditions and measures of socioeconomic status, which have demonstrated associations with poor birth outcomes.4,19,31–33

We sought to evaluate whether maternal age modifies the relationship between race and preterm birth (PTB) and its subtypes (spontaneous and clinician-initiated) by conducting analyses that incorporated universal and race-specific reference groups. We hypothesised that given population-wide changes in fertility timing, enhanced social opportunities for women, changes in obstetrical practice and heightened attention to chronic disease management, the independent effect of maternal age on PTB would not vary by race, and the risk of PTB would not diverge by race with advancing maternal age.

Methods

We conducted a population-based, cross-sectional study using the California Office of Statewide Health Planning and Development linked birth cohort data for the years 2008–2012. Using vital statistics and hospital discharge records, this linked database provides information about the maternal delivery hospitalization and maternal and infant data from birth certificates. Mother-infant pairs were linked using probabilistic linkage methods; detailed descriptions of the dataset and linkage techniques have been published previously.34 Approval for the study was obtained from Yale University Institutional Review Board, the Office of Statewide Health, Planning and Development of the State of California, the California Department of Public Health and the Committee for the Protection of Human Subjects of the State of California.

Cohort selection

There were 2,615,033 birth records in California between 2008–2012. Of these, we sequentially excluded the following: twins or higher-order births (n=83,225), stillbirths (n=4,375), and parity 2 or higher (n=1,517,044). After these exclusions, 1,007,675 singleton, live-born first births remained. We additionally excluded births with implausible maternal age and gestational age data (n=9,456), births to women of Hispanic ethnicity (n=452,356), and births to women of self-identified race other than “White” or “Black” (n=157,960), leaving the analytic cohort to 367,081 births. First births were selected to adjust for parity by design and the increasing age at first birth for women of all races in recent years.29 The cohort selection is detailed in Figure 1.

Figure 1.

Figure 1

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Selection of birth cohort for analysis, California, 2008–2012

Exposure

Maternal age was recorded at the time of delivery and categorised as <15, 15–19, 20–24, 25–29, 30–34, 35–39 and ≥40 years (range 11–71 years). Five-year age categories were chosen to reduce heterogeneity within categories and to more precisely model the shape of the relationship between exposure and outcome relative to other studies,4,19 which often used broader age categories. Maternal race/ethnicity was self-reported, abstracted from the birth certificate, and classified as non-Hispanic white (NHW) or non-Hispanic black (NHB). The sample was restricted to non-Hispanic black and white women since much of the disparities literature addresses comparisons between these two groups.35

Outcome

Gestational age was based on the last menstrual period and restricted to 22–44 weeks. Preterm birth was defined as <37 completed weeks of gestation. We approached the categorisation of this variable from a clinically-minded stand-point, and thus did not include stillbirths or those born below the limit of viability. Using a validated algorithm developed for use with US vital records, we stratified preterm births by type, categorising them as spontaneous (preceded by spontaneous membrane rupture and/or uterine contractions) or clinician-initiated (labor or delivery occurred secondary to medical intervention).36 The kappa statistic for the algorithm is 0.83 (0.70, 0.95), indicating excellent agreement between algorithm-assigned and manually-assigned preterm birth type.37

Covariate selection

Covariates were selected based on review of the literature for factors known to be associated with PTB.38–42 Sociodemographic factors included nativity (US vs foreign-born), insurance status (none, private, public), smoking status (ever/never) and age-appropriate education. An age-appropriate measure of highest level of education was used to minimise the potential effects of misclassification bias for the youngest women. Mother’s level of education was grouped as: none/not age appropriate; age appropriate (for women who were not old enough to have completed high school), high school degree or some college; associates degree; or bachelor’s degree or more. Prior to final categorization, linear contrasts were performed to ensure the homogeneity of women within each group with respect to the outcome.

All medical/obstetrical variables were categorical, and included prenatal care (none, 1st, 2nd or 3rd trimester), pre-pregnancy body mass index (BMI; underweight, normal weight, overweight, obese),43 rate of net gestational weight gain (inadequate, appropriate, excessive),44 diabetes (none, pre-pregnancy or gestational), hypertensive disorders (none, chronic/pre-pregnancy or pregnancy-associated) and asthma (yes/no). As previous studies have documented under-reporting of maternal medical conditions on the birth certificate,45 ICD-9 codes abstracted from the hospital billing records were used in conjunction with birth certificate data to define the hypertension, diabetes and asthma variables. Report of these conditions in either source was coded as presence of the condition; cross-tabulations for these variables are provided in eTable 1.

Statistical analysis

Maternal sociodemographic and health characteristics and incidence of PTB were compared by race. Multivariable Poisson regression models were used to estimate relative risk (RR) and 95% confidence intervals (95% CI) for associations with PTB, spontaneous PTB and clinician-initiated PTB, using both a universal reference group (joint effects model) and race-specific reference groups (stratified model). Statistical analysis was performed using SAS version 9.4 (SAS Institute, Cary, NC, USA).

Missing Data

Among the 2,615,033 birth records, we excluded 15 records with missing data for plurality (singleton or higher-order multiples) and 2,699 records (1%) with missing data for parity. Due to the low proportion of missingness for each key variable, records with missing or biologically implausible values for gestational age (<22 or ≥47 completed weeks, n=25,761; 2.5% missing), maternal age (n=69; <0.01% missing), race (n=20,068; 2.0% missing) or ethnicity (n=19,633; 2.0% missing) were excluded.

Although the percentage of women with missing data for any single covariate did not exceed 2.5%, we performed simple imputation by race for the nativity, insurance status, smoking status, age-appropriate education, BMI and rate of net gestational weight gain variables due to the cumulative degree of missing covariate information. Sensitivity analyses for the imputed variables demonstrated minimal change in point estimates. Imputed covariates were therefore used in all Poisson regression models. Results of the sensitivity analyses are presented in eTable 2.

Results

The final analytic sample included 367,081 singleton, live-born first births to NHW and NHB women. NHW women were more likely to be foreign-born, have an advanced degree, initiate prenatal care during the first trimester, and have a history of smoking (Table 1). NHB women were more likely to initiate prenatal care during the second trimester, be overweight or obese prior to pregnancy, exhibit a slow rate of gestational weight gain, and have hypertensive disorders of pregnancy or asthma. Women in both groups were equally likely to have health insurance, though NHW women were more likely to be privately insured and NHB women more likely to be publicly insured. Rates of pre-pregnancy diabetes, gestational diabetes and chronic hypertension were similar for both groups. The incidence of PTB was 7.7% overall; NHB women had a higher incidence than NHW women (11.1% versus 7.0%). Differences in maternal age at first birth between the two groups were evident, as more than half of NHW women gave birth to their first child between the ages of 25–34 years (55.1%), while the majority of NHB women did so between 15–24 years (63.5%).

Table 1.

Characteristics of Women in Sample Population by Race and Preterm Birth Status

Non-Hispanic Whites Non-Hispanic Blacks Total
(n=308,584, 7.0% PTB) (n=58,497, 11.1% PTB) (n=367,081, 7.7% PTB)
Number %a PTB
(%)b 		Number %a PTB
(%)b 		Number %a PTB
(%)b
 <15 140 0.05 15.7 201 0.4 21.9 341 0.1 19.4
 15–19 23,903 7.75 8.5 14,797 25.3 11.5 38,700 10.5 9.7
 20–24 66,964 21.7 6.8 22,366 38.2 10.3 89,330 24.4 7.7
 25–29 89,203 28.9 6.2 11,084 18.9 10.7 100,287 27.3 6.7
 30–34 80,815 26.2 6.6 6,313 10.8 11.6 87,128 23.7 7.0
 35–39 37,046 12.0 8.1 2,857 4.9 13.9 39,903 10.9 8.6
 ≥40 10513 3.4 10.2 879 1.5 16.0 11,392 3.1 10.7
Nativity
 US born 270,445 87.6 7.1 53,309 91.1 11.3 323,754 88.2 7.8
 Foreign born 38,139 12.4 6.5 5,188 8.9 9.3 43,327 11.8 6.8
Education
 None/not age appropriate 7,121 2.3 9.7 3,059 5.3 12.7 10,180 2.8 10.6
 High school degree, some college or age appropriate 131,478 42.6 7.6 43,719 74.7 11.3 175,197 47.7 8.5
 Associates degree 21,590 7.0 6.9 2,901 5.0 11.1 24,491 6.7 7.4
 Bachelor’s degree or more 148,395 48.1 6.4 8,818 15.1 9.8 157,213 42.8 6.6
Ever Smoker
 No 286,120 92.7 6.9 56,097 95.9 11.0 342,217 93.2 7.6
 Yes 22,464 7.3 8.0 2,400 4.1 13.4 24,864 6.8 8.5
Initiation of Prenatal care
 1st trimester 272,184 88.2 7.0 46,944 0.5 11.2 319,128 86.9 7.6
 2nd trimester 30,540 9.9 6.7 9,334 0.2 10.9 39,874 10.9 7.7
 3rd trimester 5,335 1.7 6.0 1,952 16.0 8.9 7,287 2.0 6.8
 None 525 0.2 25.9 267 3.3 28.1 792 0.2 26.6
Insurance status
 None 3,420 1.1 8.7 977 1.7 15.7 4,397 1.2 10.3
 Public 73,989 24.0 7.9 35,440 60.6 11.3 109,429 29.8 9.0
 Private 231,175 74.9 6.7 22,080 37.7 10.6 253,255 69.0 7.0
Pre-pregnancy BMI
 Underweight 14,037 4.6 8.1 2,685 4.6 13.2 16,722 4.6 8.9
 Normal weight 192,003 62.2 6.7 30,090 51.4 11.0 222,093 60.5 7.3
 Overweight 61,445 19.9 6.9 13,404 22.9 10.6 74,849 20.4 7.6
 Obese 41,099 13.3 8.3 12,318 21.1 11.6 53,417 14.5 9.1
Rate of net gestational weight gain
 Slow 70,333 22.8 7.3 16,487 28.2 11.8 86,820 23.6 8.1
 Appropriate 166,127 53.8 6.5 27,314 46.7 10.4 193,441 52.7 7.1
 Excessive 72,124 23.4 7.9 14,696 25.1 11.8 86,820 23.7 8.5
Pre-pregnancy diabetes
 No 306,542 99.3 6.9 57,917 99.0 11.0 364,459 99.3 7.6
 Yes 2,042 0.7 21.6 580 1.0 25.5 2,622 0.7 22.5
Gestational diabetes
 No 291,824 94.6 6.8 53,944 95.4 11.3 347,646 94.7 7.5
 Yes 16,760 5.4 9.9 4,553 4.6 8.9 19,435 5.3 10.6
Pre-pregnancy hypertension
 No 303,458 98.3 6.8 56,928 97.3 10.8 360,386 98.2 7.5
 Yes 5,126 1.7 16.8 1,569 2.7 24.8 6,695 1.8 18.7
Hypertensive disorders of pregnancy
 No 281,934 91.4 6.2 51,285 87.7 9.5 333,219 93.5 6.7
 Yes 26,650 8.6 15.9 7,212 12.3 22.9 33,862 9.2 17.4
Asthma
 No 293,352 95.1 6.9 53,661 91.7 11.1 347,013 94.5 7.6
 Yes 15,232 4.9 8.4 4,836 8.3 12.0 20,068 5.5 9.2
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Abbreviation: NHW, non-Hispanic white; NHB, non-Hispanic black; PTB, preterm birth

a

column percent

b

row percent

Table 2 presents results from the joint effects and stratified Poisson multivariable regression models estimating the RR of PTB, spontaneous and clinician-initiated PTB by maternal age. The joint effects model, which allows between-race comparisons using a universal reference group (NHW women aged 25–29 years), produced two parallel J-shaped curves, with a subtle widening of the difference in the risk estimates at the tails of the age distribution and narrowing at ages 20–24 years (Figure 2, panel A). Among spontaneous PTBs, the risk estimates decrease with advancing maternal age, and the difference between the estimates by race narrows (Figure 2, panel C). Conversely, the risk of clinician-initiated PTB increased dramatically for both NHB and NHW women with advancing maternal age, with risk differences initially increasing, then remaining relatively constant from age 30 onwards (Figure 2, panel E).

Table 2.

Unadjusted and adjusted relative risk estimates of preterm birth and subtypes by maternal age

Maternal
age
(years)		 Non-Hispanic White Non-Hispanic Black (Universal
reference)		 Non-Hispanic Black (Race-specific
reference)		 All Women
Rate ratio (95% confidence interval) Rate ratio (95% confidence interval) Rate ratio (95% confidence interval) Rate ratio (95% confidence interval)
Unadjusted Adjusted Unadjusted Adjusted Unadjusted Adjusted Unadjusted Adjusted
Preterm Birth
<15 2.53 (1.66, 3.84) 2.38 (1.56, 3.62) 3.52 (2.62, 4.74) 2.93 (2.17, 3.95) 2.04 (1.51, 2.76) 1.88 (1.39, 2.55) 2.88 (2.26, 3.67) 2.15 (1.68, 2.74)
15–19 1.37 (1.30, 1.44) 1.18 (1.12, 1.25) 1.86 (1.76, 1.96) 1.55 (1.46, 1.65) 1.08 (1.00, 1.16) 1.01 (0.93, 1.09) 1.44 (1.39, 1.50) 1.13 (1.08, 1.18)
20–24 1.09 (1.05, 1.13) 0.98 (0.94,1.02) 1.66 (1.58, 1.74) 1.38 (1.31, 1.45) 0.96 (0.90, 1.03) 0.91 (0.84, 0.98) 1.14 (1.10, 1.18) 0.97 (0.93, 1.00)
25–29 1.00 (reference) 1.00 (reference) 1.72 (1.62, 1.84) 1.49 (1.40, 1.59) 1.00 (reference) 1.00 (reference) 1.00 (reference) 1.00 (reference)
30–34 1.07 (1.03, 1.11) 1.14 (1.10, 1.19) 1.87 (1.73, 2.02) 1.65 (1.53, 1.78) 1.08 (0.99, 1.19) 1.12 (1.02, 1.24) 1.04 (1.00, 1.08) 1.13 (1.09, 1.17)
35–39 1.31 (1.25, 1.37) 1.37 (1.31, 1.43) 2.24 (2.02, 2.48) 1.91 (1.73, 2.12) 1.30 (1.16, 1.46) 1.31 (1.17, 1.48) 1.27 (1.22, 1.33) 1.35 (1.30, 1.41)
≥40 1.65 (1.54, 1.76) 1.63 (1.52, 1.74) 2.58 (2.18, 3.05) 2.15 (1.82, 2.54) 1.50 (1.26, 1.78) 1.46 (1.23, 1.75) 1.59 (1.50, 1.69) 1.60 (1.50, 1.70)
Spontaneous Preterm Birth
<15 2.68 (1.58, 4.53) 2.35 (1.39, 3.98) 4.60 (3.30, 6.42) 3.60 (2.57, 5.04) 2.81 (2.00, 3.95) 2.30 (1.63, 3.25) 3.57 (2.69, 4.73) 2.44 (1.84, 3.25)
15–19 1.51 (1.42, 1.61) 1.25 (1.17, 1.34) 2.12 (1.98, 2.26) 1.77 (1.64, 1.90) 1.29 (1.18, 1.42) 1.11 (1.00, 1.23) 1.63 (1.55, 1.71) 1.23 (1.16, 1.30)
20–24 1.14 (1.08, 1.20) 1.00 (0.95, 1.06) 1.70 (1.60, 1.81) 1.44 (1.35, 1.54) 1.04 (0.95, 1.14) 0.92 (0.84, 1.02) 1.19 (1.15, 1.25) 0.99 (0.95, 1.04)
25–29 1.00 (reference) 1.00 (reference) 1.64 (1.51, 1.78) 1.51 (1.39, 1.65) 1.00 (reference) 1.00 (reference) 1.00 (reference) 1.00 (reference)
30–34 1.06 (1.00, 1.11) 1.13 (1.08, 1.19) 1.64 (1.48, 1.82) 1.61 (1.45, 1.79) 1.00 (0.88, 1.13) 1.10 (0.97, 1.25) 1.03 (0.98, 1.07) 1.12 (1.07, 1.17)
35–39 1.21 (1.15, 1.29) 1.30 (1.22, 1.38) 1.64 (1.41, 1.91) 1.61 (1.38, 1.88) 1.00 (0.84, 1.18) 1.12 (0.94, 1.33) 1.16 (1.10, 1.23) 1.27 (1.20, 1.34)
≥40 1.19 (1.08, 1.32) 1.24 (1.12, 1.34) 1.54 (1.16, 2.05) 1.50 (1.13, 2.00) 0.94 (0.70, 1.26) 1.03 (0.76, 1.38) 1.14 (1.04, 1.25) 1.20 (1.10, 1.32)
Clinician-initiated Preterm Birth
<15 2.60 (1.30, 5.21) 2.66 (1.33, 5.33) 2.22 (1.15, 4.28) 1.96 (1.02, 3.78) 1.12 (0.58, 2.17) 1.19 (0.61, 2.14) 2.16 (1.34, 3.48) 1.74 (1.08, 2.81)
15–19 1.17 (1.07, 1.28) 1.05 (0.96, 1.16) 1.54 (1.40, 1.70) 1.26 (1.14, 1.40) 0.78 (0.69, 0.88) 0.80 (0.70, 0.91) 1.19 (1.11, 1.27) 0.95 (0.88, 1.03)
20–24 1.02 (0.96, 1.09) 0.94 (0.87, 1.01) 1.67 (1.54, 1.81) 1.33 (1.22, 1.45) 0.85 (0.76, 0.94) 0.86 (0.77, 0.97) 1.07 (1.01, 1.13) 0.92 (0.86, 0.97)
25–29 1.00 (reference) 1.00 (reference) 1.98 (1.79, 2.18) 1.54 (1.40, 1.71) 1.00 (reference) 1.00 (reference) 1.00 (reference) 1.00 (reference)
30–34 1.09 (1.03, 1.16) 1.17 (1.10, 1.25) 2.39 (2.13, 2.67) 1.82 (1.62, 2.04) 1.21 (1.05, 1.38) 1.17 (1.02, 1.35) 1.07 (1.01, 1.13) 1.17 (1.10, 1.24)
35–39 1.50 91.40, 1.61) 1.52 (1.42, 1.64) 3.46 (3.02, 3.97) 2.44 (2.12, 2.80) 1.75 (1.50, 2.05) 1.58 (1.34, 1.86) 1.48 (1.39, 1.58) 1.53 (1.43, 1.63)
≥40 2.48 (2.26, 2.71) 2.25 (2.05, 2.47) 4.59 (3.73, 5.65) 3.09 (2.51, 3.81) 2.32 (1.86, 2.89) 1.98 (1.58, 2.48) 2.39 (2.19, 2.59) 2.21 (2.03, 2.41)
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Universal, universal reference group (white women ages 25–29)

Race-specific, race-specific reference groups (black or white women ages 25–29, respectively)

Rate ratios were adjusted for maternal nativity, insurance status, smoking status, level of education, prenatal care, pre-pregnancy BMI, rate of net gestational weight gain, diabetes, hypertensive disorders and asthma

Figure 2.

Figure 2

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Adjusted relative risk of PTB by maternal age for NHW and NHB primiparae in California, 2008–2012. Joint effects models of (A) preterm birth, (C) spontaneous preterm birth, and (E) clinician initiated preterm birth use a universal reference group for comparisons. Stratified models of (B) preterm birth, (D) spontaneous preterm birth, and (F) clinician initiated preterm birth use race-specific reference groups for comparisons.

The stratified model used race-specific reference groups, accounting for the effect of higher baseline rates of PTB among NHB women and isolating the independent effect of maternal age by race. This model produced two similar J-shaped curves with convergence of RR from age 20 years onward (Figure 2, panel B). The confidence intervals for the RR estimates of PTB overlap at almost all maternal ages. For maternal ages 15–19 years, there was a lower relative risk of PTB for NHB (RR 1.01, 95% CI 0.93, 1.09) compared to NHW women (RR 1.18, 95% CI 1.12, 1.25).

Among NHB women, the adjusted relative risk of PTB by age was similar with overlapping confidence intervals for women aged 15–19 years (RR 1.01, 95% CI 0.93, 1.09), 20–24 years (RR 0.91, 95% CI 0.84, 0.98) and 25–29 years (reference). Among NHW women, the adjusted RR of PTB was similar for ages 20–24 years (RR 0.98, 95% CI 0.94, 1.02) and 25–29 years (RR 1.00, reference), but did not overlap and was modestly increased among women aged 15–19 years (RR 1.18, 95% CI 1.12, 1.25).

For PTB subtypes, the stratified models demonstrated decreasing risk estimates with advancing maternal age for spontaneous PTB (Figure 2, panel D), and a U-shaped curve with increasing risk estimates from age 20 years onwards for clinician-initiated PTB (Figure 2, panel F). Interestingly, there was convergence of the NHB and NHW curves across all maternal ages for spontaneous PTB, while this convergence occurs only from age 20 years and older for clinician-initiated PTB.

Table 3 presents the adjusted associations between each covariate and PTB. The highest risk of PTB (RR >2) was found among women with pre-pregnancy diabetes, hypertensive disorders of pregnancy (including pregnancy-induced hypertension, preeclampsia, eclampsia and HELLP syndrome [Hemolysis, Elevated Liver enzymes, Low Platelets]), or with no prenatal care. Women with higher levels of education (associates degree, bachelor’s degree or higher), earlier initiation of prenatal care, and who were not born in the United States had slightly decreased risk of PTB.

Table 3.

Adjusted Rate Ratios (95% confidence interval) of Preterm Birth

Race-specific
 All women
Non-Hispanic White Non-Hispanic Black
Adjusted rate ratio (95% CI) Adjusted rate ratio (95% CI) Adjusted rate ratio (95% CI)
Maternal Nativity: US born (vs foreign born) 1.05 (1.01, 1.10) 1.26 (1.14, 1.39) 1.08 (1.04, 1.13)
Maternal Education
 None/not age appropriate 1.23 (1.13, 1.33) 1.12 (1.01, 1.24) 1.19 (1.11, 1.27)
 High school degree, some college or age appropriate 1.00 (Reference) 1.00 (Reference) 1.00 (Reference)
 Associates degree 0.93 (0.88, 0.98) 0.96 (0.86, 1.08) 0.93 (0.88, 0.98)
 Bachelor’s degree or more 0.85 (0.82, 0.89) 0.83 (0.76, 0.90) 0.84 (0.82, 0.87)
Insurance status
 None 1.28 (1.14, 1.43) 1.48 (1.26, 1.75) 1.35 (1.23, 1.48)
 Public 1.14 (1.10, 1.18) 1.07 (1.01, 1.13) 1.12 (1.09, 1.15)
 Private 1.00 (Reference) 1.00 (Reference) 1.00 (Reference)
Initiation of Prenatal care
 1st trimester 1.00 (Reference) 1.00 (Reference) 1.00 (Reference)
 2nd trimester 0.91 (0.86, 0.95) 0.96 (0.90, 1.03) 0.93 (0.89, 0.96)
 3rd trimester 0.80 (0.71, 0.90) 0.77 (0.67, 0.90) 0.79 (0.72, 0.87)
 None 3.01 (2.51, 3.61) 2.25 (1.79, 2.83) 2.66 (2.31, 3.07)
Ever Smoker 1.05 (0.99, 1.10) 1.16 (1.03, 1.29) 1.07 (1.02, 1.12)
Pre-pregnancy BMI
 Underweight 1.22 (1.15, 1.30) 1.25 (1.12, 1.39) 1.23 (1.16, 1.300)
 Normal weight 1.00 (Reference) 1.00 (Reference) 1.00 (Reference)
 Overweight 0.94 (0.91, 0.98) 0.91 (0.86, 0.97) 0.94 (0.91, 0.97)
 Obese 0.95 (0.91, 0.99) 0.84 (0.79, 0.90) 0.92 (0.89, 0.95)
Rate of net gestational weight gain
 Slow 1.07 (1.04, 1.11) 1.16 (1.09, 1.23) 1.10 (1.06, 1.13)
 Appropriate 1.00 (Reference) 1.00 (Reference) 1.00 (Reference)
 Excessive 1.11 (1.08, 1.15) 1.06 (1.00, 1.13) 1.10 (1.07, 1.13)
Pre-pregnancy diabetes 2.14 (1.94, 2.36) 1.56 (1.32, 1.85) 1.96 (1.81, 2.13)
Gestational diabetes 1.20 (1.14, 1.27) 1.15 (1.04, 1.28) 1.19 (1.14, 1.25)
Pre-pregnancy hypertension 1.61 (1.49, 1.72) 1.58 (1.41, 1.76) 1.60 (1.50, 1.69)
Hypertensive disorders of pregnancy 2.35 (2.27, 2.44) 2.28 (2.15, 2.42) 2.33 (2.26, 2.41)
Asthma 1.10 (1.04, 1.17) 1.04 (0.95, 1.13) 1.08 (1.03, 1.14)
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CI, confidence interval

Rate ratios were adjusted for maternal nativity, insurance status, smoking status, level of education, prenatal care, pre-pregnancy BMI, rate of net gestational weight gain, diabetes, hypertensive disorders and asthma

Comment

Principal findings

This study shows a J-shaped relation between maternal age and the risk of PTB among singleton, live-born first births, with the highest risk of PTB among women under 15 years of age and the lowest risk among women aged 20–24 years. Stratifying age by race/ethnicity and using a universal reference group, the joint effects model demonstrated uniformly higher risk of PTB among primiparae NHB compared to primiparae NHW women, except among those younger than 15 years with clinician-initiated PTB, where the risk was higher among NHW compared to NHB women. The race-stratified model estimates, which used race-specific reference groups and adjusted for multiple covariates as in the joint effects model, produced superimposed curves which converged with advancing maternal age. The shape of the curves for PTB, spontaneous PTB and clinician-initiated PTB were distinct and maintained their respective shapes across the joint effects and stratified models (Figure 1). There is no evidence of effect modification by maternal age on the risk of preterm birth by race.

Strengths of the study

This study adds to the literature by presenting associations between maternal age and PTB, adjusted for multiple confounders and stratified by maternal race/ethnicity, using both universal and race-specific reference groups. The large sample size and ability to distinguish spontaneous from clinician-initiated preterm births facilitates a more granular examination of associations with maternal age, highlighting similarities and differences in patterns by race/ethnicity, parity, and etiology.

Limitations of the data

This study restricted the sample population to singleton first births, which was imposed to control for the potential cumulative effect of multiple previous pregnancies, prior PTB, short inter-pregnancy interval and multiple gestations, all of which are known risk factors for PTB.46,47 While this constraint permits controlling for confounding by design, it may limit the generalisability of results to women across all parity groups. The scope of the study was limited to non-Hispanic white and black women; we excluded a large percentage of births due to Hispanic ethnicity and Asian background. Because of this intentional restriction, it is unclear the extent to which these findings may be generalisable to all first births in the US.

We recognize that determination of gestational age based on last menstrual period (LMP) has some limitations, and tends to over-estimate preterm births and post-term births compared to best obstetrical estimate (OE).53 However, the percent of preterm infants within our sample closely approximated the state-specific birth data for 2013 based on OE, with the maximum discrepancy noted among infants 34–36 weeks gestational age (LMP 5.5% versus OE 6.1%). The LMP-based gestational age data was quite complete, with only 2.5% of the data missing or out of the range of biologic plausibility and demonstrated a unimodal distribution peaking at 39–40 weeks. We also note that the data was robust to multiple sensitivity analyses; together these factors lead us to believe that the likelihood of misclassification bias causing error in our point estimates is low.

Our covariates allowed us to adjust for multiple indicators of the maternal environment; however, we recognise that maternal level of education and maternal insurance status, though widely-accepted proxies for socioeconomic status, do not fully capture the intricacies of a woman’s social context and remain imperfect measures of the social environment. Moving forward, analyses incorporating neighbourhood or area-level exposures present an exciting avenue for exploration.8,54

Interpretation

This study indicates that at all maternal ages, primiparous NHB women have higher risks of PTB compared to primiparous NHW women, even after adjusting for sociodemographic and health characteristics. However, our results also suggest that the within-group risk of PTB by maternal age for NHB women is similar to the within-group risk among NHW women across all maternal age groups, and does not diverge with advancing maternal age as previously suggested.4,8,10,18,19,55 This finding suggests that the association between maternal age and the risk of PTB does not vary by race in this group of primiparous women, and the effect of maternal age does not appear to explain persistent race-disparity in PTB. However, this interpretation does not imply that maternal age and race/ethnicity are not associated with PTB. Numerous studies have demonstrated that women at the extremes of age (compared to those in their 20s) and black women (compared to white women) experience increased risk of PTB,56 which our results confirm. Rather, the findings presented here suggest that the effect of maternal age on PTB and its subtypes among primiparous women does not vary by race.

Additionally, the age-related increasing risk of PTB demonstrated in this study appears to be driven by clinician-initiated preterm births more than spontaneous preterm births and is independent of race/ethnicity. This finding speaks to differences in PTB etiology and pathophysiology by maternal age and may help identify opportunities for creating more targeted screening, intervention and prevention strategies by maternal age, regardless of maternal race/ethnicity.

Though the field of disparities research is expanding rapidly, identification of causal factors directly influencing reproductive outcomes remains elusive. Though recent studies have implicated racism and stress as etiologic factors driving disparities,35,57–60 how these concepts translate into causative biological mechanisms and pathways remains complex and poorly understood. Promising research investigating concepts of allostatic load, biological age and the role of the stress response and resiliency in relation to poor health outcomes is ongoing, but the mechanisms by which these factors affect birth timing have yet to be delineated. Contributory mechanisms are unlikely to be straight-forward, as mounting evidence implicates a complicated interplay between individual biology, environment, and social forces.

Conclusions

Moving forward, it will be important to critically evaluate the relationship between maternal age and race/ethnicity within other groups, including women of Hispanic ethnicity and Asian origin, to enrich our understanding of age-related trends in PTB and expand generalizability of these findings. Similarly, broadening analyses to include stillbirths and higher parity births will also add to the understanding of relationships between maternal age and birth outcomes. Future studies should distinguish between spontaneous and clinician-initiated preterm births, as age-based associations may not be consistent across subtypes. It will also be important to independently evaluate whether maternal age modifies the effect between race/ethnicity and other perinatal outcomes to determine whether and how etiologies, mechanisms and interventions overlap with or differ from those implicated in PTB.

Supplementary Material

Supp TableS1
Supp TableS2

Social media quote

Maternal age at delivery does not appear to drive racial disparities in preterm birth among first-time non-Hispanic white and black mothers

Figure 2

Synopsis

Study question:

Is there a differential effect of maternal age on the risk of preterm birth for non-Hispanic black compared to non-Hispanic white women?

What’s already known:

The risk of preterm birth increases with advancing maternal age, but it is unclear whether the age of highest risk and the rate at which the risk increases is the same for non-Hispanic white and non-Hispanic black women.

What this study adds:

This study suggests that among first-time mothers, the effect of maternal age on the risk of preterm birth is the same for non-Hispanic black and non-Hispanic white women, and maternal age is not the driving force behind disparities in rates of preterm birth.

Acknowledgements

The authors wish to thank Leah Horton for her assistance with data analysis, and the anonymous peer reviewers and the Editor for their valuable suggestions in revising the final manuscript. This work was partially supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH (grant T32 HD007094, AMK); and by the Sutland-Pakula Endowed Fellowship for Neonatal Research (AMK). The funding sources had no involvement in the study design; collection, analysis or interpretation of data; writing of the report, or in the decision to submit the manuscript for publication.

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Supplementary Materials

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