Socioeconomic factors and adolescent pregnancy outcomes: distinctions between neonatal and post-neonatal deaths?

Abstract

Young maternal age has long been associated with higher infant mortality rates, but the role of socioeconomic factors in this association has been controversial. We sought to investigate the relationships between infant mortality (distinguishing neonatal from post-neonatal deaths), socioeconomic status and maternal age in a large, retrospective cohort study.
We conducted a population-based cohort study using linked birth-death certificate data for Missouri residents during 1997-1999. Infant mortality rates for all singleton births to adolescent women (12-17 years, n = 10,131; 18-19 years, n = 18,954) were compared to those for older women (20-35 years, n = 28,899). Logistic regression was used to estimate adjusted odds ratios (OR) and 95% confidence intervals (CI) for all potential associations.
The risk of infant (OR 1.95, CI 1.54-2.48), neonatal (1.69, 1.24-2.31) and post-neonatal mortality (2.47, 1.70-3.59) were significantly higher for younger adolescent (12-17 years) than older (20-34 years) mothers. After adjusting for race, marital status, age-appropriate education level, parity, smoking status, prenatal care utilization, and poverty status (indicated by participation in WIC, food stamps or Medicaid), the risk of post-neonatal mortality (1.73, 1.14-2.64) but not neonatal mortality (1.43, 0.98-2.08) remained significant for younger adolescent mothers. There were no differences in neonatal or post-neonatal mortality risks for older adolescent (18-19 years) mothers.
Socioeconomic factors may largely explain the increased neonatal mortality risk among younger adolescent mothers but not the increase in post-neonatal mortality risk.

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BMC Public Health
Open Access
Research article
Socioeconomic factors and adolescent pregnancy outcomes:
distinctions between neonatal and post-neonatal deaths?
Barry P Markovitz*1,4, Rebeka Cook1, Louise H Flick1,3 and Terry L Leet1,2
Address: 1Saint Louis University School of Public Health, St. Louis, Missouri, USA, 2Saint Louis University School of Medicine, St. Louis, Missouri,
USA, 3Saint Louis University School of Nursing, St. Louis, Missouri, USA and 4Washington University School of Medicine, St. Louis, Missouri, USA
Email: Barry P Markovitz* - markovitz@wustl.edu; Rebeka Cook - rebekacook@yahoo.com; Louise H Flick - flicklh@slu.edu;
Terry L Leet - LEETTL@slu.edu
* Corresponding author
Abstract
Background: Young maternal age has long been associated with higher infant mortality rates, but
the role of socioeconomic factors in this association has been controversial. We sought to
investigate the relationships between infant mortality (distinguishing neonatal from post-neonatal
deaths), socioeconomic status and maternal age in a large, retrospective cohort study.
Methods: We conducted a population-based cohort study using linked birth-death certificate data
for Missouri residents during 1997–1999. Infant mortality rates for all singleton births to adolescent
women (12–17 years, n = 10,131; 18–19 years, n = 18,954) were compared to those for older
women (20–35 years, n = 28,899). Logistic regression was used to estimate adjusted odds ratios
(OR) and 95% confidence intervals (CI) for all potential associations.
Results: The risk of infant (OR 1.95, CI 1.54–2.48), neonatal (1.69, 1.24–2.31) and post-neonatal
mortality (2.47, 1.70–3.59) were significantly higher for younger adolescent (12–17 years) than
older (20–34 years) mothers. After adjusting for race, marital status, age-appropriate education
level, parity, smoking status, prenatal care utilization, and poverty status (indicated by participation
in WIC, food stamps or Medicaid), the risk of post-neonatal mortality (1.73, 1.14–2.64) but not
neonatal mortality (1.43, 0.98–2.08) remained significant for younger adolescent mothers. There
were no differences in neonatal or post-neonatal mortality risks for older adolescent (18–19 years)
mothers.
Conclusion: Socioeconomic factors may largely explain the increased neonatal mortality risk
among younger adolescent mothers but not the increase in post-neonatal mortality risk.
Background
Adolescent pregnancy has long been considered to
increase the likelihood of adverse infant outcomes [1].
The infant mortality rate in the United States (U.S.) for
young women has remained persistently high despite
widespread study of this phenomenon. In 1999, the
infant mortality rate for women under 20 years of age was
10.3 deaths per 1,000 live births compared to 7.0 per
1,000 live births for all ages [2] Although the U.S. teenage
(ages 15–19) birth rate has declined substantially during
the last decade, from 62.1 to 48.7 per 1,000 women in
2000 [3], this country still leads the developed countries
by wide margins. Simple calculations would suggest that,
with an estimated 470,000 births to teen mothers in the
Published: 25 July 2005
BMC Public Health 2005, 5:79 doi:10.1186/1471-2458-5-79
Received: 16 December 2004
Accepted: 25 July 2005
This article is available from: http://www.biomedcentral.com/1471-2458/5/79
© 2005 Markovitz et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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year 2000, over 1,500 excess infant deaths occurred
among adolescent mothers.
The etiology explaining the higher mortality risk for ado-
lescent mothers has been debated for as long as the phe-
nomenon has been observed. Miller argued that
socioeconomic status has a direct effect on infant mortal-
ity, confounding the role of young maternal age, in a
study of 2,019 U.S. counties from 1971–1975 [4]. Many
studies point towards low birth weight (including infants
born preterm and those small-for-gestational age) as the
predominant proximate marker of infant mortality [5-
10]. Fraser, in a large study spanning 20 years of live births
from Utah, found maternal age to be an independent pre-
dictor for poor birth outcomes, but this study did not spe-
cifically address infant mortality or socioeconomic status
directly [11]. The defining influence of the biology of ado-
lescent pregnancy, risky adolescent behavior, nutrition,
race, socioeconomics, and adequacy of prenatal care – or
the interaction among these variables – remains contro-
versial. Certainly poverty, minority status, suboptimal
prenatal care, poor education and unmarried status are
more common in teenage compared to older mothers,
and are known risk factors for low birth weight, preterm
birth and higher infant mortality in the United States [11].
Adolescent mothers are disproportionately poor. For
example, in Missouri for 1997–1999, 79% of births to
women aged 14–19 were covered by Medicaid compared
to only 35.7% of births to women 20 and older [12].
Among adolescents living in poor communities, 69%
report having had intercourse, compared to 37% of those
living in more affluent neighborhoods [13]. Support can
be found to defend or refute all or none of these factors as
independent risk factors [6,7,11,14-16].
We examined linked birth-death certificate data for Mis-
souri residents to assess the role of poverty status and
other potential risk factors on infant mortality among
teenage mothers. We hypothesized that teen motherhood
is highly confounded by poverty, and that there is no
independent effect of adolescent age on infant mortality.
Our analysis distinguished neonatal and post-neonatal
mortality among younger and older adolescent mothers.
Methods
Study design
This was a population-based cohort study using Mis-
souri's linked birth-death certificate files for 1997–1999.
This period was chosen since it provided a sufficiently
large study population with the most current birth-related
data. To test the primary hypothesis, we compared infant
mortality rates of adolescent and older mothers while
controlling for socioeconomic status (defined by partici-
pation in poverty assistance programs) and other known
and purported risk factors.
Cohort description
The study population included all singleton live births for
mothers who were Missouri residents and less than 20
years of age during 1997–1999. The study population was
stratified by maternal age to identify younger (12–17
years) and older (18–19 years) adolescent mothers. A
comparison population included a random sample of sin-
gleton live births for mothers who were 20–34 years old
during 1997–1999, since the infant mortality rates were
relatively stable for this age group but higher for older
mothers. Women other than non-Hispanic white or Afri-
can American were excluded, because they comprised less
than 3% of all live births in Missouri during the study
period.
Covariates
The following covariates were extracted from the birth cer-
tificate: race/ethnicity (non-Hispanic black or non-His-
panic white), maternal education level (recorded as age
appropriate based on the number of years of formal edu-
cation completed by the mother), parity, marital status,
tobacco or alcohol use (as dichotomous and categorical
covariates based on the number of cigarettes per day or
drinks per week), maternal weight gained during preg-
nancy, adequacy of prenatal care utilization (based on the
Kotelchuck index)[17], and socioeconomic status. The
poorest women were defined as those receiving food
stamps, since this program includes only those with per-
sonal incomes less than 130% of the federal poverty level.
Poor women were those defined as receiving Medicaid or
WIC. In Missouri, pregnant women at or below 185% of
the federal poverty level are eligible for Medicaid and
WIC. While personal incomes can be used to identify
women participating in poverty programs, we cannot
assume that all non-participants are above 185% of the
federal poverty level. All other women included those
who did not participate in any of the three poverty assist-
ance programs. Other covariates were clinical estimate of
gestational age (20–28, 29–32, 33–36, ≥37 weeks) and
birth weight (<2500, ≥ 2500 grams). Infant mortality
included deaths that occurred during the neonatal (0–27
days) and post-neonatal (28–364 days) period. The
underlying causes of death were divided into five catego-
ries: 1) accidental, 2) respiratory arrest or sudden infant
death syndrome, 3) infectious, 4) perinatal or congenital,
5) other deaths.
Analysis
Chi-square tests and student's unpaired t-tests were used
to determine if categorical and continuous covariates,
respectively, were significantly different for younger ver-
sus older mothers. Overall infant mortality, as well as neo-
natal mortality and post-neonatal mortality, was assessed
as a function of the three age cohorts and stratified by race.
Logistic regression was used to assess how each covariate

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affected the role of maternal age on infant mortality risk.
Covariates that altered the odds ratio (OR) for the primary
relationship by more than 10% were considered signifi-
cant and included in the full model. 95% confidence
intervals (CI) were computed to estimate the precision of
the OR. First-order interaction terms between each covari-
ate and maternal age were tested in a secondary analysis.
The significance of adding each interaction was tested
with a p-value <0.01. The final model was examined for
outliers, influential data and goodness of fit (Omnibus
Test and Hosmer and Lemeshow test, p <0.05). The role of
each covariate in the full model was also assessed using
neonatal and post-neonatal mortality as the primary out-
come in separate logistic regression analyses. SPSS (Chi-
cago, IL, version 10.0) was used for all statistical analyses.
This research, reviewed by the Saint Louis University insti-
tutional review board, was classified as exempt from the
U.S. Department of Health and Human Services regula-
tions for the protection of human subjects. The exemption
45 CFR 46.101(b)(4) permits epidemiologic research that
uses existing publicly available data that are maintained
in such a manner that subjects cannot be identified
directly or through identifiers linked to the subjects.
Results
The distribution of covariates and outcomes for the three
maternal age cohorts is reported in Table 1. Comparing all
adolescent mothers (12 to 19 years) to older mothers,
newborns of older mothers were, on average, 167 grams
heavier than newborns of younger mothers (p < 0.001).
Although the younger mothers gained 1.36 kg (3 pounds)
on average more during pregnancy than older mothers (p
< 0.001), this is not likely to be clinically significant.
Compared to older mothers, younger mothers were more
likely to be nulliparous (72.6% vs. 29.1%), smoke (27.2%
vs. 18.2%), of black race (26.7% vs. 13.8%), and on Med-
icaid (78% vs. 34.8%), WIC (75.1% vs. 36%), or food
stamps (25.5% vs. 16.4%). Not surprisingly, fewer
younger mothers were married (20.2% vs. 72.3%) and
more had inadequate prenatal care (16.8% vs. 7%).
Younger mothers were less likely to have an age-appropri-
ate educational level (72.6% vs. 86.8%), and more likely
to have a preterm infant (9.8% vs. 7.6%).
Table 1: Characteristics of Missouri women with singleton live births, 1997–1999, by maternal age.
12 to 17 (N = 10,131)18 to 19 (N = 18,954) 20–35 (N = 28,899)
CharacteristicN%N%N%
Infant mortality
Neonatal mortality
Post-neonatal mortality
Clinical Estimate of Gestational Age
121
69
52
40
102
161
808
8,961
9,114
1,017
137
6,688
3,306
8,592
7,698
7,761
2,060
8,660
9,140
2,486
1.2
0.7
0.5
0.4
1.0
1.6
8.0
89.0
90.0
10.0
1.4
66.0
32.6
86.1
77.1
77.8
20.7
85.5
90.3
24.7
0.6
4.8
20.7
52.0
22.4
158
92
65
50
110
252
1,318
17,109
17,358
1,596
256
14,234
4,464
12,516
14,983
14,085
5,351
12,375
14,064
5,425
0.8
0.5
0.3
0.3
0.6
1.3
7.0
90.8
91.6
8.4
1.4
75.1
23.6
66.0
80.4
75.6
28.7
65.3
74.3
28.7
0.5
3.9
14.7
58.7
22.7
173
111
60
59
139
260
1,739
26,571
27,207
1,692
635
24,288
3,976
25,086
10,070
10,412
4,729
8,382
7,986
5,262
268
805
2,037
18,832
7,225
0.6
0.4
0.2
0.2
0.5
0.9
6.0
92.4
94.1
5.9
2.2
84.0
13.8
86.8
35.4
36.7
16.7
29.0
27.7
18.3
0.9
2.8
7.0
65.2
25.0
<24 Weeks
24–28 weeks
29–32 weeks
33–36 weeks
>36 weeks
Normal (> 2500 gm)
Low (≤ 2500 gm)
Missing/other
Non-Hispanic white
Non-Hispanic black
Birth Weight Category
Maternal Race
Age appropriate education level
Medicaid
W.I.C.
Food Stamp Program
Parity
Unmarried
Tobacco use During Pregnancy
Alcohol use during pregnancy
Kotelchuck index
nulliparous
6190
Unknown
Inadequate
Adequate/Intermediate
Adequate Plus
487
2,101
5,273
2,270
747
2,789
11,121
4,297
p ≤ 0.001 for all comparisons

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Overall, infants born of the youngest mothers (12 to 17
years) were 1.69 (CI 1.24, 2.31) times more likely to die
during the neonatal period and 2.47 (CI 1.70, 3.59) times
more likely during the post-neonatal period compared to
older mothers Table 2 illustrates the risk of neonatal mor-
tality and post-neonatal mortality for the adolescent age
cohorts compared to older mothers, stratified by race.
There is an increased risk of infant (9.7 deaths/1000 live
births; OR 1.95, CI 1.42, 2.67), neonatal (5.3 deaths/1000
live births; 1.63, CI 1.02, 2.47) and post-neonatal mortal-
ity (4.4 deaths/1000 live births; OR 2.58, CI 1.56, 4.24)
for the 12–17 year old non-Hispanic white mothers com-
pared to older mothers (5.0, 3.2 and 1.7 infant, neonatal
and post-neonatal deaths/1000 live births respectively).
Among non-Hispanic white mothers 18–19 years of age,
there is an increased risk of infant (6.7 deaths/1000 live
births; OR 1.35, CI 1.02, 1.79) and post-neonatal (3.0
deaths/1000 live births; OR 1.75, CI 1.11, 2.75) mortality,
but not neonatal mortality (3.7 deaths/1000 live births;
OR 1.14, CI 0.79, 1.64). No association between age and
infant, neonatal or post-neonatal mortality was seen for
non-Hispanic black mothers. However, the crude rates of
infant, neonatal and post-neonatal mortality for non-His-
panic black mothers were approximately twice those for
non-Hispanic white mothers across all age groups.
Table 2: Infant, neonatal and post-neonatal mortality risk by age group, stratified by race
RaceAge group Neonatal mortalityPost-neonatal mortality
Rate (per 1000 births)cOR (95% CI) Rate (per 1000 births)cOR (95% CI)
Non-Hispanic White12–17 years
18–19 years
20–35 years
5.3
3.7
3.2
1.63 (1.02, 2.47)
1.14 (0.79, 1.64)
reference
4.4
3.0
1.7
2.58 (1.56, 4.24)
1.75 (1.11, 2.75)
reference
Non-Hispanic Black12–17 years
18–19 years
20–35 years
10.4
9.0
7.3
1.41 (0.84, 2.39)
1.23 (0.74, 2.04)
reference
7.0
5.0
4.3
1.63 (0.84,3.19)
1.15 (0.59, 2.26)
reference
cOR = crude odds ratio, CI = confidence intervals
Table 3: Logistic regression models: effect of maternal age on infant mortality.
Neonatal Mortality Post-Neonatal Mortality
Maternal Age (years)aOR 95% CIaOR95% CI
12–17
18–19
20–34
Non-Hispanic black
Age-inappropriate educational Level
Unmarried
Nulliparous
Tobacco Use
Kotelchuck Index
Unknown
Inadequate
Adequate
Adequate Plus
Poverty Status
Poorest *
Poor **
Other
1.43
1.15
1.00
2.08
1.39
1.50
1.00
1.49
0.98, 2.08
0.83, 1.60
reference
1.56, 2.78
1.01, 1.90
1.06, 2.14
0.99, 1.02
1.10, 2.01
1.73
1.04
1.00
1.45
1.73
1.25
1.01
1.26
1.14, 2.64
0.71, 1.53
reference
1.02, 2.04
1.23, 2.44
0.83, 1.88
0.99, 1.03
0.89, 1.79
4.04
1.98
1.00
2.89
2.53, 6.45
1.36, 2.88
reference
2.17, 3.85
0.88
1.58
1.00
1.89
0.35, 2.21
1.05, 2.39
reference
1.35, 2.66
0.36
0.45
1.00
0.25, 0.53
0.33, 0.62
reference
2.29
1.60
1.00
1.36, 3.86
0.98, 2.61
reference
cOR = crude odds ratio, aOR = adjusted odds ratio, CI = confidence intervals
* defined as participation in food stamps
** defined as participation in WIC or Medicaid but not food stamps

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Table 3 includes the results of the logistic regression
model. Race, appropriate education level, marital status,
parity, smoking during pregnancy, level of prenatal care
and participation in Medicaid, WIC, or food stamp pro-
grams significantly affected the primary effect of maternal
age on infant mortality risk and were included in the final
model. Controlling for these variables, the risk for neona-
tal and post-neonatal mortality was 1.43 (CI 0.98, 2.08)
and 1.73 (CI 1.14, 2.64), respectively, for the youngest
adolescent mothers. There was no increased risk of
neonatal or post-neonatal mortality for older adolescent
mothers. Furthermore, adjusting for gestational age did
not appreciably change any of the adjusted OR for the pri-
mary effect of age on infant mortality. First-order
interactions between covariates and age, considered as a
block, did not reach significance and were dropped from
subsequent analyses.
Of particular interest was the differential effect of the pov-
erty construct in the model on neonatal and post-neona-
tal mortality risks. Poverty was defined by participation in
Medicaid, WIC and/or food stamp programs, and the
effect of such programs appeared to be strongly protective
against neonatal mortality when controlling for maternal
age, but a significant risk factor for post-neonatal mortal-
ity. Mothers participating in the food stamps program
(our "poorest" category) had a reduced risk of neonatal
mortality (OR 0.36, CI 0.25, 0.53) compared to mothers
who did not participate in any of the poverty assistance
programs. However, their risk of post-neonatal mortality
(OR 2.29, CI 1.36, 3.86) was significantly higher. Stratify-
ing the post-neonatal deaths by cause of death appears to
show a higher percentage of deaths due to accidents or
infections for the youngest mothers compared to older
mothers in our study population (Table 4).
Discussion
We conclude that neonatal mortality rates do not differ by
maternal age, but post-neonatal rates may be higher for
mothers aged 12–17 years
socioeconomic and other known risk factors. Our results
after adjusting for
are most similar to Reichman and Pagnini, who studied
all New Jersey births in 1989 and 1990 [18]. They found
that controlling for medical and socioeconomic factors
accounted for some, but not all the effect of young mater-
nal age on infant mortality. The unadjusted odds ratio of
infant mortality among white 15–17 year olds in their
study was 2.5; the adjusted value was 1.6. Their study was
strengthened by access to linked uniform billing hospital
discharge records in addition to state vital statistics. They
were therefore able to assess socioeconomic status much
more directly in their model, comparing Medicaid recipi-
ents and "self-pay" to the privately insured.
In our study, we were limited to inferring poverty status by
the dichotomous values of participation in WIC, Medic-
aid, or food stamps programs as noted on the birth certif-
icate. Recognizing the eligibility criteria for these
programs (130% of federal poverty level for food stamps
and less than 185% of poverty level for WIC and Medic-
aid) allowed us to predict confidently income levels of
program participants, but we cannot conclude that those
who are not participants are of higher income levels. Par-
ticularly in the aftermath of the Personal Responsibility
and Work Opportunity Reconciliation Act of 1996, many
individuals who were eligible may not have enrolled in
such programs. Nevertheless, 75–80% of the two younger
maternal cohorts were enrolled in Medicaid and/or WIC,
compared to only 36% of the older mothers. Such dra-
matic differences are likely to be truly related to socioeco-
nomic distinctions and not bureaucratic or regulatory
obstacles unique to older women. This study is also lim-
ited by the validity of the original data entered on birth
and death certificates. Of particular concern would be the
possibility of differential bias in reporting of information
between older and younger mothers since there is evi-
dence that reporting infant mortality may suffer from a
racial bias and the race distributions are not identical in
our age cohorts [19].
Our finding that participation in WIC and Medicaid
appears to be protective against neonatal mortality is con-
Table 4: Underlying causes of post-neonatal death by maternal age (n = 180).
Cause of death (%)
Maternal age
(years)
Accidental n (%) SIDS/
respiratory
arrest n (%)
Perinatal/
congenital n (%)
Infectious n (%) All other n (%)Total n (%)
12–17
18–19
20–35
8(15)
11 (17)
4(6)
16(31)
34 (52)
22 (35)
12 (23)
9(14)
16 (26)
4(8)
1(2)
2(3)
12 (23)
11 (17)
18 (29)
52 (100)
66 (100)
62 (100)
chi-square = 14.3, p = 0.08