Association Between Changes in Maternal Age Distribution and Hypertensive Disorders of Pregnancy in the United States, 2011-2021

Abstract

Background:

Hypertensive disorders of pregnancy (HDP) are associated with increased long-term risk of both maternal and offspring cardiovascular disease. The incidence of HDP has increased concurrently with the upward shift in maternal age distribution; however, the association between changes in maternal age distribution and rates of HDP in the United States is unknown.

Methods:

Data from 2011-2021 were abstracted from the National Center for Health Statistics (NCHS), which includes all live births in the United States. Births to individuals aged 15-44 years were included given that 99% of all births occur in this reproductive age group. HDP were coded as “yes”/”no” and included both gestational hypertension and preeclampsia. We described changes in HDP per 1,000 live births between 2021-2011 by calculating rate ratios (RR). We then decomposed the change in frequency of HDP with the Kitagawa technique into the two rate change components: 1) change in maternal age distribution, and 2) change in age-specific HDP rates (i.e., the incidence rate at a given age).

Results:

In 2011 and 2021, there were 7,595,018 live births that occurred. The mean maternal age increased from 27.9 (6.1) to 29.9 (5.7) years. The frequency of HDP increased significantly from 43.8 (43.6, 44.0) to 90.9 (90.6, 91.2) per 1,000 live births from 2011 to 2021. The Kitagawa decomposition revealed that the change in maternal age distribution had no association with the increased rate of HDP (0.0 [−0.1, 0.1]). Conversely, the age-specific rate of HDP increased across all age groups and was responsible for the total change in HDP (47.1 [46.7, 47.4]).

Conclusions:

Although both maternal age and frequency of HDP increased significantly between 2011 and 2021, the higher rates of HDP were not due to the upward shift in the age distribution, but rather, to increases in HDP frequency within all age groups.

INTRODUCTION

Hypertensive disorders of pregnancy (HDP) is a well-established risk factor for the development of long-term cardiovascular disease for both the mother as well as their offspring.^1–3 Despite advances in both obstetric and cardiovascular care, the incidence of HDP doubled from 2007 to 2019. Concurrently, the mean age of pregnant individuals has also increased.⁴ Because older maternal age is associated with a higher risk of HDP, the secular rise in maternal age may be an important contributor to the adverse trends in rates of HDP.⁵ Nevertheless, whether and to what degree the upward shift in the age distribution of birthing individuals is associated with changes in the HDP rate is not known. In order to inform public health strategies for prevention of HDP, we sought to investigate the association between temporal changes in the age distribution of birthing individuals and age-specific HDP rates in the US between 2011-2021.

METHODS

We performed a serial, cross-sectional analysis using maternal data from the National Center for Health Statistics (NCHS) Natality Files, which includes all live births in the US. We included data from all live births to individuals aged 15-44 years from 2011 to 2021. Consistent with prior publications, we selected births to individuals aged 15-44 years given that 99% of all births occur in this reproductive age group.^6–8 All data and materials are publicly available at (https://www.cec.gov/nchs/data_access/vitalstatsonline.thm). Within the NCHS dataset, new-onset HDP is coded as “yes” or “no” and includes either gestational hypertension or preeclampsia without pre-pregnancy hypertension. Maternal race and ethnicity, which are social constructs, were based on self-report on the birth certificate data. Urbanization was categorized as “urban” or “rural” using the National Center for Health Statistics Urban-Rural Classification Scheme. Given this study included individuals who had >1 pregnancy resulting in a live birth during the study period, we performed a sensitivity analysis restricted to first live birth from 2011 – 2021. An additional sensitivity analysis was performed to assess the effects of the COVID pandemic on the results.

HDP rate ratios (RR) and respective 95% confidence intervals were calculated for each age stratum overall and represented the number of cases of HDP per 1,000 population in 2021 for every 1 case of HDP per 1,000 population in 2011. Maternal age was stratified into 5-year strata (15-19, 20-24, 30-34, 35-39, 40-44 years) and RR for each age group was calculated. Kitagawa decomposition analysis^9,10 was utilized to partition the temporal changes in HDP rates into two components: 1) changes due to shifts in maternal age distribution, and 2) changes due to shifts in age-specific rates. The two components were calculated relative to each other; one was held constant as the observed variation in the other component was assessed. Any rate or ratio, including HDP rates, can be considered the product of the percentage distribution of a factor (maternal age) and factor-specific ratios (maternal age-specific HDP rates). For each age stratum, the decomposition components are calculated as follows: Let P_2011i = the proportion of the ith age strata in 2011 and P_2021i = the proportion of the ith age strata in 2021. Let R_2011i = the HDP rate in 2011 for the ith age strata and R_2021i = the HDP rate in 2021 for the ith age strata. The formulas are as follows:

$$DistributionComponent=\sum _{i=1}^5(\left(P_{2021i}-P_{2011i}\right)\times \frac{R_{2021i}+R_{2011i}}{2})$$

$$RateComponent=\sum _{i=1}^5(\left(R_{2021i}-R_{2011i}\right)\times \frac{P_{2021i}+P_{2011i}}{2})$$

The sum of the age distribution and age-specific rate components across the age groups over time equaled the total difference over time in the HDP rate.

Bootstrapping was used to calculate 95% confidence intervals (CIs) for age distribution and age-specific rate components. Given known socially-mediated differences in maternal age at delivery and HDP frequency by race and ethnicity and county-level urbanization, all analyses were repeated stratified by race and ethnicity as well as urban versus rural residence. Analyses were performed with R 4.2.2, all tests were two-tailed, and p<0.05 was used to define statistical significance. This study was considered exempt from review by the Northwestern institutional review board, given that all data were publicly available and de-identified.

RESULTS

Of the 7,595,018 live births that occurred from 2011 and 2021, inclusive, to individuals aged 15-44 years in the US; 54% were to non-Hispanic White individuals (NHW), 15% were to non-Hispanic Black individuals (NHB), 23% were to Hispanic individuals, 6% were to Asian American or Pacific Islanders (AAPI), and 1% were to American Indian or Alaskan Natives (AIAN). From 2011 to 2021, the mean (standard deviation) maternal age increased from 27.9 (6.1) to 29.9 (5.7) years. Among maternal age strata, declines in live births occurred among 15-19-year-olds (−51.9% [−52.2%, −51.6%]), 20-24-year-olds (−24.4% [−24.6%, −24.2%]), and 25-29-year-olds (−2.0% [−2.3%, −1.8%]). Increases occurred among 30-34-year-olds (22.0% [21.7%, 22.2%]), 35-39-year-olds (37.8% [37.3%, 38.3%]), and 40-44-year-olds (25.2% [24.2%, 26.3%]).

The overall HDP rate approximately doubled from 43.8 (43.6, 44.0) to 90.9 (90.6, 91.2) per 1,000 live births (RR 2.07 [2.06, 2.09]) over the study period. Further, the rate of HDP increased every year with no significant inflection point in its annual percent change (Table 1). The decomposition analysis partitioned the overall HDP rate difference of 47.1 per 1,000 live births into two components: age distribution and age-specific rates (Table 2). The upward shift in age distribution was not responsible for any of the temporal rise in HDP rates (0.00 (−0.06, 0.05) per 1,000 live births). Instead, the increase in HDP was solely attributed to rate increases in the age-specific groups. Individuals aged 15-19 (2.8 [2.7, 2.9]), 20-24 (10.3 [10.1, 10.4]), 25-29 (13.2 [13.0, 13.4]), 30-34 (12.5 [12.3, 12.7]), 35-39 (6.7 [6.5, 6.8]), and 40-44 (1.6 [1.5, 1.7]) years saw significant increases in age-specific HDP rates, with relative rate changes greater for younger patients (Figure 1).

Table 1.

Annual Change in Live Births and Hypertensive Disorders of Pregnancy Rates – United States 2011-2021

	2011	2012	2013	2014	2015	2016	2017	2018	2019	2020	2021
Live Births, N	3,942,006	3,941,412	3,920,911	3,976,864	3,967,072	3,934,579	3,844,260	3,780,401	3,736,144	3,602,653	3,653,012
HDP, N	172,799	180,714	188,475	200,980	221,510	234,514	247,091	270,323	290,200	302,907	332,116
HDP* Rate (95% CI)	43.8 (43.6, 44.0)	45.8 (45.7, 46.0)	48.1 (47.9, 48.3)	50.5 (50.3, 50.8)	55.8 (55.6, 56.1)	59.6 (59.4, 59.8)	64.3 (64.0, 64.5)	71.5 (71.3, 71.8)	77.7 (77.4, 77.9)	84.1 (83.8, 84.3)	90.9 (90.6, 91.2)
HDP Rate Ratio (95% CI)	Reference	1.05 (1.04 - 1.05)	1.10 (1.09 - 1.10)	1.15 (1.15 - 1.16)	1.27 (1.27 - 1.28)	1.36 (1.35 - 1.37)	1.47 (1.46 - 1.48)	1.63 (1.62 - 1.64)	1.77 (1.76 - 1.78)	1.92 (1.91 - 1.93)	2.07 (2.06 - 2.09)

Definition of Abbreviations: HDP = Hypertensive Disorders of Pregnancy

^*Per 1,000 live births to individuals aged 15 – 44 years.

Table 2.

Change in Age Distribution, Hypertensive Disorders of Pregnancy Rates, and Components of Overall Hypertensive Disorders of Pregnancy – United States 2011-2021

	Change in Live Births (95% CI), %	Average Annual Percent Change in HDP (95% CI)	Absolute Change in HDP 2011-2021 (95% CI)	Age-Distribution Change (95% CI)	Age-Specific Change (95% CI)
Overall	NA	8.0 (7.4, 8.7)	47.1 (46.7, 47.4)	0.0 (−0.1, 0.1)	47.1 (46.7, 47.4)
Age, years
15 - 19	−51.9 (−52.2, −51.6)	7.6 (6.8, 8.4)	−0.2 (−0.3, −0.1)	−3.0 (−3.04, −2.96)	2.8 (2.7, 2.9)
20 - 24	−24.4 (−24.6, −24.2)	8.6 (7.7, 9.7)	6.4 (6.2, 6.5)	−3.9 (−3.9, −3.8)	10.3 (10.1, 10.4)
25 - 29	−2.0 (−2.3, −1.8)	8.1 (6.7, 9.6)	12.8 (12.6, 13.0)	−0.4 (−0.4, −0.3)	13.2 (13.0, 13.4)
30 - 34	22.0 (21.7, 22.2)	8.0 (7.5, 8.6)	16.1 (15.9, 16.3)	3.6 (3.5, 3.6)	12.5 (12.3, 12.7)
35 - 39	37.8 (37.3, 38.3)	7.6 (7.3, 7.9)	9.8 (9.6, 9.9)	3.1 (3.1, 3.1)	6.7 (6.5, 6.8)
40 - 44	25.2 (24.2, 26.3)	6.7 (6.1, 7.4)	2.2 (2.1, 2.3)	0.6 (0.6, 0.6)	1.6 (1.5, 1.7)

Definition of Abbreviations: HDP = Hypertensive Disorders of Pregnancy; CI = Confidence Interval

For Total Effect on HDP Rate, Age-Distribution Change, and Age-Specific Change, the sum of each individual maternal age group contribution is equal to the overall contribution.

Per 1,000 live births to individuals aged 15 – 44 years. For each age stratum, the decomposition components are calculated as follows: Let P_2011i = the proportion of the ith age strata in 2011 and P_2021i = the proportion of the ith age strata in 2021. Let R_2011i = the HDP rate in 2011 for the ith age strata and R_2021i = the HDP rate in 2021 for the ith age strata. The formulas are as follows: $\mathrm{The\; age\; distribution}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{n}\mathrm{t}=\sum _{\mathrm{i}=1}^5(\left({\mathrm{P}}_{2021\mathrm{i}}-{\mathrm{P}}_{2011\mathrm{i}}\right)\times \frac{{\mathrm{R}}_{2021\mathrm{i}}+{\mathrm{R}}_{2011\mathrm{i}}}{2})$. $\mathrm{T}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{g}\mathrm{e}-\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{n}\mathrm{t}=\sum _{\mathrm{i}=1}^5(\left({\mathrm{R}}_{2021\mathrm{i}}-{\mathrm{R}}_{2011\mathrm{i}}\right)\times \frac{{\mathrm{P}}_{2021\mathrm{i}}+{\mathrm{P}}_{2011\mathrm{i}}}{2})$.

Figure 1. Rate Ratios for Hypertensive Disorders of Pregnancy by Maternal Age.

Rate ratios represent the number of hypertensive disorders of pregnancy per 1,000 population in 2021 for every 1 event of hypertensive disorders of pregnancy per 1,000 population in 2011 for birthing individuals aged 15 – 44 years.

When the analysis was further stratified by maternal race and ethnicity, similar trends to the overall were observed, with HDP rates more than doubling for all racial and ethnic subgroups (Figure 2). The majority of HDP rate increases for all racial and ethnic subgroups were attributable to increases in age-specific rates (Table 3 and Supplemental Figure S1). When stratified by urban or rural residence, the findings were again similar to the overall analysis (Table 4), with increases in HDP, regardless of urbanicity, that were wholly due to increases in age-specific rates of HDP.

Figure 2. Rate Ratios for Hypertensive Disorders of Pregnancy by Maternal Race and Ethnicity.

Definition of Abbreviations: NHW = Non-Hispanic White; NHB = Non-Hispanic Black; AAPI = Asian American or Pacific Islanders; AIAN = American Indian or Alaskan Native

Rate ratios represent the number of hypertensive disorders of pregnancy per 1,000 population in 2021 for every 1 event of hypertensive disorders of pregnancy per 1,000 population in 2011 for birthing individuals aged 15 – 44 years.

Table 3.

Change in Age Distribution, Hypertensive Disorders of Pregnancy, and Components of Hypertensive Disorders of Pregnancy by Race and Ethnicity – United States 2011-2021

	Hypertensive Disorders of Pregnancy				Rate Change Components
	2011, N*	2011 HDP Rate†	2021, N	2021 HDP Rate*	Absolute Change in HDP 2011-2021 (95% CI)	Age-Distribution Change (95% CI)	Age-Specific Rate Change (95% CI)
NHW	102,044	47.6 (47.4, 48.0)	181,984	96.7 (96.2, 97.1)	49.0 (48.5, 49.4)	−0.5 (−0.5, −0.4)	49.5 (49.0, 49.9)
NHB	31,520	54.4 (53.8, 54.9)	56,376	109.3 (108.5, 110.1)	54.9 (53.9, 56.0)	0.7 (0.5, 0.9)	54.3 (53.2, 55.3)
Hispanic	30,266	33.1 (32.70, 33.44)	66,332	75.1 (74.6, 75.6)	42.0 (41.4, 42.7)	0.5 (0.4, 0.6)	41.6 (40.9, 42.2)
AAPI	6,055	25.3 (24.6, 25.9)	13,536	60.9 (59.9, 61.9)	35.6 (34.4, 36.8)	1.1 (0.8, 1.3)	34.6 (33.4, 35.7)
AIAN	2,103	53.8 (51.6, 55.7)	2,827	108.5 (104.8, 112.2)	54.7 (50.3, 58.6)	2.4 (1.6, 3.2)	52.3 (48.1, 56.6)

Definition of Abbreviations: HDP = Hypertensive Disorders of Pregnancy; NHW = Non-Hispanic White; NHB = Non-Hispanic Black; AAPI = Asian American or Pacific Islanders; AIAN = American Indian or Alaskan Native

^*811 HDP births in 2011 did not have race reported. In 2016, the National Center for Health Statistics began reporting race consistent with the 1997 Office of Management and Budget standards. As such 11,061 HDP births in 2021 with race listed as more than one race, unknown or not stated, or not available were not included.

^†Adverse pregnancy outcome per 1,000 live births to individuals aged 15 – 44 years. For each age stratum, the decomposition components are calculated as follows: Let P_2011i = the proportion of the ith age strata in 2011 and P_2021i = the proportion of the ith age strata in 2021. Let R_2011i = the HDP rate in 2011 for the ith age strata and R_2021i = the HDP rate in 2021 for the ith age strata. The formulas are as follows: $\mathrm{T}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{n}\mathrm{t}=\sum _{\mathrm{i}=1}^5(\left({\mathrm{P}}_{2021\mathrm{i}}-{\mathrm{P}}_{2011\mathrm{i}}\right)\times \frac{{\mathrm{R}}_{2021\mathrm{i}}+{\mathrm{R}}_{2011\mathrm{i}}}{2})$. $\mathrm{T}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{n}\mathrm{t}=\sum _{\mathrm{i}=1}^5(\left({\mathrm{R}}_{2021\mathrm{i}}-{\mathrm{R}}_{2011\mathrm{i}}\right)\times \frac{{\mathrm{P}}_{2021\mathrm{i}}+{\mathrm{P}}_{2011\mathrm{i}}}{2})$.

Table 4.

Change in Age Distribution, Hypertensive Disorders of Pregnancy, and Components of Hypertensive Disorders of Pregnancy by Urbanization Category – United States 2011-2021

	Adverse Pregnancy Outcomes				Rate Change Components
	2011, N	2011 HDP Rate*	2021, N	2021 HDP Rate*	Age-Distribution Change	Age-Specific Rate Change	Total Effect
Urban	145,272	42.7 (42.5, 42.9)	284,629	90.1 (89.8, 90.5)	0.02 (−0.04, 0.07)	47.4 (47.1, 47.8)	47.4 (47.1, 47.8)
Rural	27,527	50.9 (50.3, 51.5)	47,487	95.9 (95.0, 99.6)	0.3 (0.2, 0.5)	44.6 (43.5, 45.6)	44.9 (43.9, 45.9)

^*Adverse pregnancy outcome per 1,000 live births to individuals aged 15 – 44 years. For each age stratum, the decomposition components are calculated as follows: Let P_2011i = the proportion of the ith age strata in 2011 and P_2021i = the proportion of the ith age strata in 2021. Let R_2011i = the HDP rate in 2011 for the ith age strata and R_2021i = the HDP rate in 2021 for the ith age strata. The formulas are as follows: $\mathrm{T}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{n}\mathrm{t}=\sum _{\mathrm{i}=1}^5(\left({\mathrm{P}}_{2021\mathrm{i}}-{\mathrm{P}}_{2011\mathrm{i}}\right)\times \frac{{\mathrm{R}}_{2021\mathrm{i}}+{\mathrm{R}}_{2011\mathrm{i}}}{2})$. $\mathrm{T}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{n}\mathrm{t}=\sum _{\mathrm{i}=1}^5(\left({\mathrm{R}}_{2021\mathrm{i}}-{\mathrm{R}}_{2011\mathrm{i}}\right)\times \frac{{\mathrm{P}}_{2021\mathrm{i}}+{\mathrm{P}}_{2011\mathrm{i}}}{2})$.

DISCUSSION

The frequency of HDP increased along with maternal age at delivery from 2011 to 2021. However, the upward shift in age distribution did not underlie the increase in HDP, either for the overall population or in racial/ethnic or residential-area subgroups. Furthermore, our findings demonstrate that over the last decade, younger birthing individuals have experienced greater relative rate increases in HDP compared with older birthing individuals. Understanding why the frequency of HDP has increased in every age group – and disproportionately so in younger individuals – is a critical next step to improve maternal health in the United States.

Our results defining the impact of age distribution on national trends in rates of HDP extends upon recent work demonstrating an acceleration of adverse pregnancy outcomes, maternal cardiovascular comorbidities, and pregnancy-related cardiovascular complications over the last decade.^11,12 Furthermore, the results support that the primary driver of increasing HDP rates is not due to an increase in the average age of birthing individuals, but rather due to increases in age-specific rates. Recent increases in obesity, pre-diabetes, pre-pregnancy hypertension, and diabetes among younger individuals have contributed to a greater prevalence of suboptimal pre-pregnancy cardiovascular health and may be associated with the increased age-specific rates seen for HDP.^13–16

In terms of relative changes over time, individuals aged 20-24 years saw the greatest increase in rates of HDP from 2011 - 2021. Furthermore, there was a stepwise decline in the overall increase in HDP rate as age strata became progressively older. The finding that HDP rates are increasing most rapidly among younger individuals is of particular interest. This mirrors recent data demonstrating an inverse relation between the increase in rates of pre-pregnancy obesity and maternal age.¹⁷ The rate of pre-pregnancy hypertension has also been shown to have a greater temporal increase among birthing individuals aged 20-24 years than their older counterparts.¹³ This prior work illustrating more rapid increases in risk factors for HDP among younger individuals highlights the need for public health interventions that target pre-pregnancy health in order to prevent further increases in HDP.

In addition to pre-pregnancy risk factors such as obesity, diabetes, and chronic hypertension, older age – often categorized by advanced maternal age (AMA) – is another well-established risk factor for HDP.^18,19 However, those who were AMA saw a less dramatic rise in HDP rates compared to their younger counterparts. Those who are of older maternal age may have different approaches to care, such as higher rates of screening and maternal-fetal-medicine consults.²⁰ In addition to this, AMA is considered a moderate risk factor to be considered in the determination of low-dose aspirin use for the prevention of preeclampsia, which may further lower rates of HDP in these age groups.²¹

There are two main research implications that arise after our study. First, given that HDP risk is largely not attributable to increasing maternal age, this study should encourage further work to understand the underpinnings of the increased incidences of HDP to determine potential targets for intervention. Second, given the relative greater rate increases seen among younger individuals, further work is needed to identify clinical and public health efforts to mitigate further worsening of maternal outcomes in these age strata.

Due to the use of the NCHS Natality Files, which includes all live births within the US, our study utilizes the largest and most complete dataset available to analyze the temporal rates of HDP. Additionally, forms for NCHS Natality Files are completed using a standardized protocol by healthcare professionals present at the time of delivery.²² This approach allows high-quality and reliable data compared to other datasets that capture HDP based on self-report alone. While NCHS Natality files are the largest and most comprehensive dataset for HDP, there is the potential for misclassification on birth registration. The sensitivity of HDP recorded on birth certificates ranges from approximately 30 – 60%, however, the specificity is high (99%). Therefore, we may have underestimated the incidence of HDP in this study.²³ Given that HDP was defined as an aggregate of gestational hypertension and preeclampsia, we are unable to differentiate the rates of these two diseases. Another potential limitation is that the potential for greater awareness of HDP diagnosis or changes in coding may have influenced the results. However, these trends for awareness and coding reflect underlying changes in cardiovascular risk factors outside of pregnancy for obesity and chronic hypertension. Given the deidentified nature of this publicly available dataset, repeat pregnancies leading to live births were included in the estimation of rates. However, in a sensitivity analysis that analyzed only nulliparous individuals, the findings were similar to that seen in for all live births. An additional sensitivity analysis was performed for the years 2019 – 2021 that showed rate changes during the COVID pandemic were similar to that of the overall study period. Lastly, our study is ecological in nature and therefore cannot identify causes for the trends observed. While rising rates in younger age groups likely are due to multifactorial reasons, the current study statistical approach addressed the focused question whether age was associated with the change. Therefore, we applied the Kitagawa decomposition statistical technique, which only allows for the rate to be decomposed into two components. However, changes in other key risk factors such as obesity and pre-pregnancy hypertension are important considerations for future research.

In summary, rates of HDP have increased substantially since 2011. The overall increased rate of HDP was not attributable to the upward shift in maternal age but was rather due to the increases in age-specific rates, with a relatively greater increase among younger individuals. Further research is needed to identify what potentially modifiable factors are contributing to the increases in age-specific rates as well as to identify public health interventions to improve maternal health and outcomes.

Clinical Perspective.

What is Known

• Hypertensive Disorders of pregnancy have more than doubled over the last 2 decades. Concurrently, the mean age of pregnant individuals has also increased in the United States. Older maternal age is associated with a higher risk of hypertensive disorders of pregnancy.

What The Study Adds

• The upward shift in age distribution in the United States did not underlie the increase in Hypertensive Disorders of pregnancy for the overall population, or racial/ethnic or residential-area subgroups.

Non-standard Abbreviations and Acronyms

AAPI

Asian American or Pacific Islander

AIAN

American Indian or Alaskan Native

AMA

Advanced Maternal Age

HDP

hypertensive disorders of pregnancy

NCHS

National Center for Health Statistics

NHB

Non-Hispanic Black

NHW

Non-Hispanic White