Supplemental Analyses of Recent Trends in Infant Mortality
by
Kenneth D. Kochanek, M.A., and Joyce A. Martin, M.P.H.
Overview
Preliminary
data for 2002 indicate an increase in the infant mortality rate (IMR) to
7.0 deaths per 1,000 live births from 6.8 in 2001 (1) (table 1). To better
understand the increase in the IMR for 2002, the Centers for Disease
Control and Prevention’s National Center for Health Statistics (NCHS)
reviewed additional partially edited mortality records processed after the
close of the preliminary data file as well as partially edited fetal death
data for 2002 (processed as of January 2004). These data indicate that the increase in the 2002 IMR observed for
preliminary data will be confirmed by final data, and that while the IMR
increased, the perinatal mortality rate (late fetal deaths plus early
neonatal deaths per 1,000 live births plus fetal deaths) remained stable.
(Final statistics will be available later this year.)
The
rise in the IMR is concentrated in the neonatal period (0-27 days),
particularly in the early neonatal period (0-6 days) (tables 1 and 2). The
rate of late fetal mortality (fetal deaths of 28 or more weeks of
gestation per 1,000 live births plus fetal deaths) shows a 3 percent
decline for 2002 (slightly greater than the average annual decline for
1990-2001). As a result, the
perinatal mortality rate, which more fully describes the risk of death at
late stages of pregnancy and shortly after birth, appears unchanged for
2002 (table 2 and figure 1). The
perinatal mortality rate had declined fairly consistently for more than
half of a century.
Cause of death
One
way to better understand the increase in the IMR is to examine changes by
cause using final 2001 and preliminary 2002 data. When examined by cause of infant death, 3 causes among the 10
leading causes of infant death appear to account for most of the current
year increase: Congenital malformations, deformations and chromosomal
abnormalities; Disorders related to short gestation and low birth weight,
not elsewhere classified; and Newborn affected by maternal complications
of pregnancy (table 3). Historically,
the majority of these deaths have been among infants born low birthweight
(weight at delivery of less than 2,500 grams or 5 and one-half pounds).
Despite steady increases in preterm and low birthweight rates between 1990
and 2002, trends over this time period for these three causes do not
indicate a consistent pattern of increase or decrease (2-14). However, the interpretation of the cause of death trends is
complicated by the change in the ICD revision in 1999 that created breaks
in the comparability of cause of death statistics (5).
Historical trends in infant
mortality
The
death registration area was completed in 1933 when all States were
included in the collection of death statistics (15). Since then, with the exception of
1957-58, when a significant increase
in the IMR was observed, the historical trend of the IMR has been one of
steady, sometimes rapid decline. Through
the 1930s and 1940s, the IMR declined by an average of 4 percent per year.
The rate of decline slowed markedly to 1 percent per year for 1950
to 1964. Thereafter, until the
early 1980s, infant mortality declined rapidly, by an average of almost 5
percent per year (16). From
1981 to 1989 the rate of decline again slowed to an average of 2 percent
per year (3).
Recent trends in fetal and
infant mortality
Over
the more recent period, 1990 to 2001, the IMR declined 26 percent (from
9.2 to 6.8 per 1,000) for an average decrease of 3 percent per year (3)
(table 1 and figure 2). Between 1990 and 2001 the neonatal mortality rate
declined from 5.8 to 4.5 per 1,000 (down 22 percent) and the postneonatal
mortality rate from 3.4 to 2.3 (down 32 percent) (3). Between 1990 and 2001, the late fetal mortality rate declined
fairly steadily, by 23 percent, from 4.3 to 3.3 per 1,000 (17) (table 2).
The perinatal mortality rate also declined steadily between 1990 and 2001,
from 9.1 to 6.9 for a total of 24 percent (table 2 and figure 2). Although
the pace of decline has slowed somewhat since the mid-1990s, significant declines in late fetal mortality
and infant mortality have been observed through 2001 despite substantial
increases in preterm and low birthweight risk, two important predictors of
perinatal health.
As discussed above,
preliminary data for 2002 indicate
a 3 percent rise in the IMR from 2001. The increase was observed for neonatal deaths only; a 4-percent
increase in the neonatal mortality rate was reported, whereas the
postneonatal rate remained constant. The increase in neonatal mortality
was accompanied by a 3-percent decline in the late fetal mortality rate
and, as a result, the perinatal mortality rate was unchanged for 2002.
In
addition to releasing preliminary and final mortality and natality
statistics, NCHS also releases monthly provisional counts
of vital events (as differentiated from records
of vital events) and corresponding rates along with cumulative provisional
information for the past 12 months and for all months to date in the
calendar year. The provisional series is subject to reporting
inconsistencies with considerable variation in the resulting estimates
(18). However, the provisional
series can be used to get early information on more recent trends with the
understanding that the findings may change when more complete data are
available. NCHS
currently has provisional counts of infant deaths for the first 9 months of 2003. These counts
suggest an improvement in the IMR between 2002 and 2003; however, the
provisional data are not stable enough to determine if the magnitude of
that improvement is large enough to bring the rate down to or below the
historically low level reached in 2001.
Potential explanatory
factors for the changes in the infant mortality rate
Changes
in the characteristics of births and changes in birthweight and
gestation-specific infant mortality rates (i.e., the death rate for
infants at a given weight or gestational age) may be related to changes in
the IMR. Final birth data for
2002 indicate that the two key predictors of infant health, the percent of
births born preterm (less than
37 completed weeks of gestation) and low birthweight (LBW), continued
to climb, rising 1 to 2
percent for 2002 (2) (table 2
and figure 2). Increases in
preterm and LBW rates of 3 and 1 percent respectively, were also noted
between 2000 and 2001. Since
1990 preterm and LBW rates have risen fairly steadily, preterm by 14
percent (from 10.6 to 12.1 percent) and LBW by 11 percent (from 7.0 to 7.8
percent).
The
bulk of the increase has been among moderately preterm (32-36 weeks of
gestation) and moderately low birthweight (1,500-2,499 grams) infants.
Between 1990 and 2002, the moderately preterm rate rose from 8.7 to 10.1
percent and the moderately low birthweight (MLBW) rate from 5.7 to 6.4
percent, whereas the very preterm rate (less than 32 weeks of gestation) rose from
1.92 to 1.96 percent and the very low birthweight rate (VLBW) (less than
1,500 grams) from 1.27 to 1.46 percent.
Although still at increased risk compared with term or normal
birthweight infants, infants born moderately preterm and MLBW, are at
substantially lower risk than their very preterm and VLBW counterparts for
early death. For 2001, 18
percent of infants born very preterm did not survive the first year of
life compared with less than 1 percent of infants born moderately preterm
(19).
Multiple
births, more than half of which are born preterm and/or low birthweight,
have contributed importantly to recent increases in preterm and LBW rates.
Between 1990 and 2002 the multiple birth rate climbed 42 percent (a
3-percent rise was reported between 2001 and 2002) (2, 20); in 2002 nearly
one-fourth of all LBW infants were born in a multiple delivery (2). Multiple
births do not account for all of the preterm/LBW rise; however, the
preterm rate for singletons alone increased 7 percent over this period
(2). (While the rate of moderately preterm singleton births rose from 8.01
to 8.87 percent between 1990 and 2002, the very preterm rate for
singletons declined slightly, from 1.69 to 1.57 percent.)
The
increased use of assisted reproductive therapies (ART) such as in-vitro
fertilization has been strongly associated with the growth in multiple
gestation pregnancies (21) and may also be associated with an increased
risk of LBW among singletons (22,23). One percent of all 2001 births were
the result of ART procedures (24).
Changes in the management of labor and delivery influenced at least in
part by the increased use of medical technologies (e.g., ultrasound) (25),
and more aggressive management of premature rupture of the membranes
(PROM) (26,27) may also be related to the trends in preterm/LBW births as
induction of labor and cesarean delivery occur more often at earlier
gestational ages; the use of induction of labor and of cesarean delivery
among births delivered preterm has risen substantially in recent years (28-30).
Recent declines in infant mortality have been attributed to improvement
in birthweight and gestation-specific infant mortality rates, not to the
prevention of preterm or LBW births (31). The decline in birthweight and gestation-specific mortality has
been attributed primarily to improvements in obstetric and neonatal care
such as pulmonary surfactants for preterm infants. Data from the 2002 linked birth and infant death file, to be
available in the summer of 2004, will allow us to assess whether changes
in birthweight and gestation-specific mortality rates have contributed to
the current year rise in the IMR.
Summary
Preliminary
data for 2002 show a significant increase in the IMR to 7.0 infant deaths
per 1,000 live births, the first rise in the U.S. IMR since 1958 (1,15).
Review of additional partially edited 2002 mortality data indicate
that the increase in the IMR will be confirmed by final data. The 2002
increase in infant mortality was concentrated in the neonatal period,
particularly in deaths occurring within 7 days of birth. Partially edited fetal death data suggest that the increase in
neonatal mortality was accompanied by a decline
in the late fetal mortality rate for 2002, and thus it appears that the
2002 perinatal mortality rate will remain level.
Data
from the 2002 linked birth and infant death file, which are expected to be
available by the summer of 2004, will allow us to assess the contribution
of maternal and infant factors such as multiple births and management of
labor and delivery. With this file, it will also be possible to
investigate whether changes in birthweight and gestation-specific neonatal
mortality rates relate to the increase in infant mortality.
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Definition of
terms
Infant mortality rate –
Deaths of infants aged under 1 year per 1,000 or 100,000 live births. The
infant mortality rate is the sum of the neonatal and postneonatal
mortality rates.
Neonatal mortality rate –
Deaths of infants aged 0-27 days per 1,000 live births. The neonatal
mortality rate is the sum of the early neonatal and late neonatal
mortality rates.
Early neonatal mortality
rate – Deaths of infants aged 0-6 days per 1,000 live births.
Late neonatal mortality rate
– Deaths of infants aged 7-27 days per 1,000 live births.
Postneonatal mortality rate
– Deaths to infants aged 28 days-1 year per 1,000 live births.
Late fetal mortality rate
– Fetal deaths of 28 or more weeks of gestation per 1,000 live births
plus fetal deaths.
Perinatal mortality rate –
Late fetal deaths plus early neonatal deaths per 1,000 live births plus
fetal deaths.
Low birthweight rate –
Births with weight at delivery of less than 2,500 grams per 100 live
births. The low birthweight
rate is the sum of the moderately low and very low birthweight rates.
Moderately low birthweight
rate – Births with weight at delivery of 1,500-2,499 grams per 100 live
births.
Very low birthweight rate
– Births with weight at delivery of less than 1,500 grams per 100 live
births.
Term – Births at 37-41
weeks of gestation.
Preterm rate – Births at
less than 37 completed weeks of gestation per 100 live births. The preterm
rate is the sum of the moderately and very preterm rates.
Moderately preterm rate –
Births at 32-36 weeks of gestation per 100 live births.
Very preterm rate – Births
at less than 32 weeks of gestation per 100 live births.
Acknowledgments
The authors would like to acknowledge the assistance
of Jennifer H. Madans, Office of the Director; Charles J. Rothwell,
Director of the Division of Vital Statistics; James A. Weed, Acting Chief,
Robert N. Anderson, Lead Statistician, and Betty L. Smith of the Mortality
Statistics Branch; Stephanie J. Ventura, Chief, T. J. Mathews and Paul D.
Sutton of the Reproductive Statistics Branch; Steven J. Steimel and Jaleh
Mousavi of the Systems, Programming and Statistical Resources Branch; and
David Justice of the Data Acquisition and Evaluation Branch.
Table 1. Infant deaths and infant, neonatal, and postneonatal mortality rates: United States, 1933, 1940, 1950, 1960, 1970, and 1975-2001 final, and 2002 (preliminary and latest processed)
[Rates per 1,000 live births]
Year
Infant
deaths
Infant
mortality rate
Total
Neonatal
(under 28 days)
Postneonatal
(28 days - 11 months)
Total
Under
7 days
7-27 days
20021
28,042
7.0
4.7
3.7
0.9
2.3
20022
27,977
7.0
4.7
…
…
2.3
2001
27,568
6.8
4.5
3.6
0.9
2.3
2000
28,035
6.9
4.6
3.7
1.0
2.3
1999
27,937
7.1
4.7
3.8
1.0
2.3
1998
28,371
7.2
4.8
3.8
1.0
2.4
1997
28,045
7.2
4.8
3.8
0.9
2.5
1996
28,487
7.3
4.8
3.8
0.9
2.5
1995
29,583
7.6
4.9
4.0
0.9
2.7
1994
31,710
8.0
5.1
4.2
0.9
2.9
1993
33,466
8.4
5.3
4.3
0.9
3.1
1992
34,628
8.5
5.4
4.4
1.0
3.1
1991
36,766
8.9
5.6
4.6
1.0
3.4
1990
38,351
9.2
5.8
4.8
1.0
3.4
1989
39,655
9.8
6.2
5.1
1.1
3.6
1988
38,910
10.0
6.3
5.2
1.1
3.6
1987
38,408
10.1
6.5
5.4
1.1
3.6
1986
38,891
10.4
6.7
5.6
1.1
3.6
1985
40,030
10.6
7.0
5.8
1.2
3.7
1984
39,580
10.8
7.0
5.9
1.1
3.8
1983
40,627
11.2
7.3
6.1
1.2
3.9
1982
42,401
11.5
7.7
6.4
1.3
3.8
1981
43,305
11.9
8.0
6.7
1.3
3.9
1980
45,526
12.6
8.5
7.1
1.4
4.1
1979
45,665
13.1
8.9
7.5
1.4
4.2
1978
45,945
13.8
9.5
8.0
1.5
4.3
1977
46,975
14.1
9.9
8.4
1.5
4.2
1976
48,265
15.2
10.9
9.3
1.6
4.3
1975
50,525
16.1
11.6
10.0
1.6
4.5
1970
74,667
20.0
15.1
13.6
1.5
4.9
1960
110,873
26.0
18.7
16.7
2.0
7.3
1950
103,825
29.2
20.5
17.8
2.7
8.7
1940
110,984
47.0
28.8
23.3
5.5
18.3
19333
120,887
58.1
34.0
26.3
7.7
24.1
… Data not available. 1 Partially edited data processed through January 2004. 2 Preliminary data processed though July 10, 2003. 3 First year in which all States were included in the collection of death statistics.
SOURCE: Various CDC/NCHS publications; see text.
Table 2. Trends in measures of perinatal health: United States, 1990-2002
Year
Preterm1
Low birthweight2
Number
Percent
Number
Percent
Total
Very preterm1
Moderately perterm1
Total
Very low birthweigth2
Moderately low birthweight2
2002
480,812
12.1
2.0
10.1
314,077
7.8
1.5
6.4
2001
476,250
11.9
1.9
10.0
308,747
7.7
1.4
6.2
2000
467,201
11.6
1.9
9.7
307,030
7.6
1.4
6.1
1999
460,853
11.8
2.0
9.8
301,183
7.6
1.5
6.2
1998
452,275
11.6
2.0
9.6
298,208
7.6
1.4
6.1
1997
436,600
11.4
1.9
9.4
291,154
7.5
1.4
6.1
1996
423,107
11.0
1.9
9.1
287,230
7.4
1.4
6.0
1995
424,455
11.0
1.9
9.1
285,152
7.3
1.3
6.0
1994
431,613
11.0
1.9
9.1
287,607
7.3
1.3
6.0
1993
435,625
11.0
1.9
9.1
288,482
7.2
1.3
5.9
1992
430,239
10.7
1.9
8.8
287,493
7.1
1.3
5.8
1991
440,082
10.8
1.9
8.9
292,230
7.1
1.3
5.8
1990
436,590
10.6
1.9
8.7
289,418
7.0
1.3
5.7
Year
Late
fetal mortality3
Early neonatal mortality4
Perinatal
mortality5
Number
Rate
Number
Rate
Number
Rate
20026
12,871
3.2(3.19)
15,003
3.7
27,874
6.9
2001
13,251
3.3(3.28)
14,611
3.6
27,862
6.9
2000
13,506
3.3
14,913
3.7
28,419
7.0
1999
13,427
3.4
14,889
3.8
28,316
7.1
1998
13,474
3.4
15,073
3.8
28,547
7.2
1997
13,686
3.5
14,859
3.8
28,545
7.3
1996
14,078
3.6
14,968
3.8
29,046
7.4
1995
14,251
3.6
15,515
4.0
29,766
7.6
1994
14,781
3.7
16,523
4.2
31,304
7.9
1993
15,297
3.8
17,395
4.3
32,692
8.1
1992
16,782
4.1
17,798
4.4
34,580
8.5
1991
17,010
4.1
18,916
4.6
35,926
8.7
1990
17,959
4.3
20,020
4.8
37,979
9.1
1 Less than 37 completed weeks of gestation. Very preterm is less than 32 completed weeks of gestation. Moderately preterm is 32-36 completed weeks of gestation.
2 Less than 2,500 grams. Very low birthweight is less than 1,500 grams. Moderately low birthweight is 1,500-2,499 grams.
3 The late fetal mortality rate = fetal deaths with stated or presumed gestational ages of 28 weeks or more per 1,000 live births plus late fetal deaths.
4 Deaths to infants less than 7 days per 1,000 live births.
5 Late fetal deaths plus infant deaths less than 7 days per 1,000 live births plus late fetal deaths.
6 Partially edited data processed as of January 2004.
SOURCE: Various CDC/NCHS publications; see text.
Table
3. Infant deaths and infant mortality rates for all causes and infant
mortality rates by leading causes: United States, preliminary 2002, and
final 1990-2001
[Rates per 100,000 live births]
ICD-101
ICD-92,3
Cause
of death (Based on the International Classification of Diseases,
Tenth Revision, 1992)
20024
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
All
causes (deaths)
27,977
27,568
28,035
27,937
28,371
28,045
28,487
29,583
31,710
33,466
34,628
36,766
38,351
All
causes (rates)
696.1
684.8
690.7
705.6
719.8
722.6
732.0
758.6
802.2
836.6
851.9
894.4
922.3
Congenital
malformations, deformations and chromosomal abnormalities (Q00-Q99)
140.7
136.9
141.5
138.2
157.6
159.2
164.0
168.1
173.4
178.2
183.2
186.9
198.1
Disorders
relating to short gestation and low
birth weight, not elsewhere classified (P07)
114.4
109.5
108.3
110.9
104.0
101.1
100.3
100.9
107.6
107.7
99.3
100.7
96.5
Sudden
infant death syndrome (R95)
50.6
55.5
62.2
66.9
71.6
77.1
78.4
87.1
103.0
116.7
120.3
130.1
130.3
Newborn
affected by maternal complications of
pregnancy (P01)
42.9
37.2
34.6
35.3
34.1
32.1
32.1
33.6
32.8
33.6
35.9
37.4
39.8
Newborn
affected by complications of placenta,
cord and membranes (P02)
25.3
25.3
26.2
25.9
24.4
24.7
24.4
24.7
24.0
24.8
24.4
23.4
23.4
Respiratory
distress of newborn (P22)
23.8
25.1
24.6
28.0
32.9
33.5
35.0
37.3
39.6
45.4
50.8
62.5
68.5
Accidents
(unintentional injuries) (V01-X59)
22.2
24.2
21.7
21.3
18.3
19.0
20.0
19.5
21.7
21.6
19.2
22.3
21.8
Bacterial
sepsis of newborn (P36)
18.3
17.3
18.9
17.5
18.7
18.4
17.6
18.4
18.7
17.2
19.8
19.3
19.2
Diseases
of the circulatory system (I00-I99)
16.1
15.4
16.3
16.8
25.2
24.7
24.3
24.8
24.1
23.6
23.2
22.8
24.5
Intrauterine
hypoxia and birth asphyxia (P20-P21)
14.4
13.3
15.5
15.5
11.7
11.6
11.0
12.2
13.6
13.7
15.1
14.6
18.3
1 World Health Organization. International Statistical Classification of Diseases and Related Health Problems, Tenth Revision. Geneva: World Health Organization. 1992.
2 World Health Organization. Manual of the International Statistical Classification of Diseases, Injuries, and Causes of Death, based on the recommendations of the Ninth Revision Conference, 1975. Geneva: World Health Organization. 1977. 3 Cause of death titles in ICD-9 differ in some cases from those in ICD-10. Breaks in the comparability of some causes listed have resulted in changes in category titles and from coding rules used to select the underlying cause of death. Comparisons between 1998 and 1999 infant mortality rates by cause of death in this table should not be made. The trends displayed should be interpreted separately as trends from 1990-1998 and 1999-2002. For a complete explanation of differences and measurement of the discontinuities between ICD-9 and ICD-10, see reference number 5 from the text section of this
Health E-Stat. 4 Preliminary data for 2002 processed through July 10, 2003.
SOURCE: Various CDC/NCHS publications; see text.
