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Kogan MD, Martin JA, Alexander GR, Kotelchuck M, Ventura SJ, Frigoletto FD. The Changing Pattern of Prenatal Care Utilization in the United States, 1981-1995, Using Different Prenatal Care Indices. JAMA. 1998;279(20):1623–1628. doi:10.1001/jama.279.20.1623
From the Centers for Disease Control and Prevention, National Center for Health Statistics, Hyattsville, Md (Dr Kogan and Mss Martin and Ventura); the Department of Maternal and Child Health, University of Alabama at Birmingham (Dr Alexander); the Department of Maternal and Child Health, University of North Carolina at Chapel Hill (Dr Kotelchuck); and the Division of Obstetrics, Harvard Medical School, Massachusetts General Hospital, Boston (Dr Frigoletto).
Context.— Two measures traditionally used to examine adequacy of prenatal care
indicate that prenatal care utilization remained unchanged through the 1980s
and only began to rise slightly in the 1990s. In recent years, new measures
have been developed that include a category for women who receive more than
the recommended amount of care (intensive utilization).
Objective.— To compare the older and newer indices in the monitoring of prenatal
care trends in the United States from 1981 to 1995, for the overall population
and for selected subpopulations. Second, to examine factors associated with
receiving intensive utilization.
Design.— Cross-sectional and trend analysis of national birth records.
Setting.— The United States.
Subjects.— All live births between 1981 and 1995 (N=54 million).
Main Outcome Measures.— Trends in prenatal care utilization, according to 4 indices (the older
indices: the Institute of Medicine Index and the trimester that care began,
and the newer indices: the R-GINDEX and the Adequacy of Prenatal Care Utilization
Index). Multiple logistic regression was used to assess the risk of intensive
prenatal care use in 1981 and 1995.
Results.— The newer indices showed a steadily increasing trend toward more prenatal
care use throughout the study period (R-GINDEX, intensive or adequate use,
32.7% in 1981 to 47.1% in 1995; the Adequacy of Prenatal Care Utilization
Index, intensive use, 18.4% in 1981 to 28.8% in 1995), especially for intensive
utilization. Women having a multiple birth were much more likely to have had
intensive utilization in 1995 compared with 1981 (R-GINDEX, 22.8% vs 8.5%).
Teenagers were more likely to begin care later than adults, but similar proportions
of teens and adults had intensive utilization. Intensive use among low-risk
women also increased steadily each year. Factors associated with a greater
likelihood of receiving intensive use in 1981 and 1995 were having a multiple
birth, primiparity, being married, and maternal age of 35 years or older.
Conclusions.— The proportion of women who began care early and received at least the
recommended number of visits increased between 1981 and 1995. This change
was undetected by more traditional prenatal care indices. These increases
have cost and practice implications and suggest a paradox since previous studies
have shown that rates of preterm delivery and low birth weight did not improve
during this time.
PRENATAL CARE is one of the most frequently used health services in
the United States according to data from the 1995 National Medical Ambulatory
Care Survey.1 The National Medical Ambulatory
Care Survey estimated that there were 23.3 million prenatal visits to either
physicians' offices, outpatient departments, or emergency departments in 1995.1 Moreover, after general medical examination, prenatal
care is the most frequently cited preventive health service.1
have indicated the positive benefits of prenatal care use. Based in part on
these reports, national programs and policies, such as the Year 2000 Objectives
for the United States, have been initiated to increase prenatal care access
Accurate measurement of prenatal care utilization is critical in assessing
the need for health services, monitoring health care utilization trends, and
understanding the relationship between prenatal care services and pregnancy
outcomes. This is particularly true for the 1980s and 1990s, when myriad changes
occurred in the practice, organization, and reimbursement of prenatal care.
Diagnostic and treatment procedures, such as ultrasound and amniocentesis,
became more prevalent.17 Medicaid expansions
increased eligibility for prenatal care and reimbursed a wider array of prenatal
services, while enrolling more women into managed care.18-20
Further, in 1989, the US Public Health Service's Expert Panel on the Content
of Prenatal Care recommended fewer, but more comprehensive, prenatal visits
for low-risk women.21 Yet, despite these developments,
the 2 measures that have traditionally been used for monitoring national trends
in prenatal care utilization and examining the impact of health services on
pregnancy outcomes, ie, the trimester that care began and the Institute of
Medicine (IOM) Index (also called the Kessner Index),22
indicated that prenatal care utilization remained unchanged through the 1980s
and only began to rise slightly in the 1990s.23-25
Both the trimester that care began and the IOM Index have been criticized
as painting an incomplete or inaccurate picture of prenatal care utilization.10,26,27 The trimester that
care began does not take into account subsequent prenatal care visits. For
example, a woman who began care in the first trimester but had no more visits
and a woman who began care in the first trimester and had 13 regularly spaced
visits would be similarly classified.26 The
IOM Index misclassifies prenatal care utilization adequacy among women who
had a pregnancy of more than 36 weeks' gestation.26
For these women, it requires only 9 visits to be "adequate," while the American
College of Obstetricians and Gynecologists (ACOG) recommends more than 9 visits.28
In recent years, 2 other indices have been developed to correct for
some of the shortcomings of the IOM Index.10,26,27
These indices, the R-GINDEX and the Adequacy of Prenatal Care Utilization
(APNCU) Index, are both based on the full ACOG recommendations and include
"no prenatal care" and "intensive utilization" categories.10,26,27
Research has indicated that women who receive more than the recommended amount
of care (intensive use) are a distinct group and should be studied independently.10,29
While several studies have compared the newer indices with the older
indices, none is national in scope,27,30,31
nor have they examined how these indices compare in monitoring changes over
time. While national trends in prenatal care utilization are believed to have
been relatively stable for many years, these trends may reflect the limited
sensitivity of the trimester and IOM Index measures and not reflect actual
prenatal care utilization itself.
The purpose of this study is to compare these older and newer indices
in the monitoring of trends in prenatal care utilization in the United States
from 1981 to 1995, both in terms of the overall population and in 3 selected
subpopulations: women who had a multiple birth, teenagers (<18 years old),
and women considered at low risk based on sociodemographic characteristics.
The data used were drawn from the National Center for Health Statistics'
natality files for 1981 to 1995. More than 54 million birth records were available
for analysis. The number of births annually ranged from 3629238 to 4158212.
Birth records with inconsistent or missing values for the month prenatal care
began, the number of prenatal visits, or the length of gestation were excluded
from the analysis in the calculation of each index. Records with 1 or more
missing values for gestational age, month care began, or number of visits
ranged from 5.0% to 6.8% for each year.
Information on the month prenatal care began is measured from the date
the last normal menses began and may be obtained from the mother, physician,
or hospital record. The number of prenatal visits
is defined as "visits made for medical supervision of the pregnancy by a physician
or other health care provider."32 It is obtained
from the same sources and is not intended to include encounters where only
screening procedures such as amniocentesis or ultrasound are performed.
Gestational age in completed weeks is computed from the interval between
the first day of the last normal menstrual period and the date of birth. Records
missing the date of the last normal menstrual period are imputed when there
is a valid month and year. Imputation procedures have been described in detail
In this study we examine 4 measures of prenatal care utilization over
time, the trimester that care began, the IOM Index, the R-GINDEX, and the
APNCU Index. The IOM Index, published in 1973, was derived in part from the
prenatal care visit schedule recommended by ACOG,28
although it did not accurately follow their guidelines.22,26
All of the prenatal care measures, except the trimester that prenatal care
began are based on the month that care began, and the number of visits, adjusted
for gestational age, and include the categories "adequate," "intermediate,"
"inadequate," and "no care." For comparability, a separate "no prenatal care"
classification was added to each measure based on the categorization used
in the R-GINDEX.27 The trimester that care
began categorizes women only according to when they began prenatal care.
The R-GINDEX and the APNCU Index also contain an "intensive use" category.26,27 The "intensive use" category for
the R-GINDEX includes women who had an excessively large number of prenatal
care visits (approximately 1 SD beyond the mean number of visits for women
initiating care within each trimester). The APNCU Index compares the actual
number of visits with the expected number based on the full ACOG recommendations,
adjusted for the onset of prenatal care and gestational age at delivery.26 The intensive group for the APNCU Index consists
of women who have observed-to-expected ratios of at least 110% of the ACOG-recommended
Even though the R-GINDEX and the APNCU Index both use the same ACOG
criteria for recommended visits, each index categorizes utilization differently
depending on when care began and, for intensive use, the threshold number
of visits. Therefore, for example, "adequate" on one index is not analogous
to "adequate" on another index.27Table 1 presents an example of how each
index would classify the prenatal care utilization of women who give birth
at 32 or 40 weeks of gestation. Detailed descriptions of these indices are
We examined 3 groups considered to be at higher or lower risk of adverse
pregnancy outcomes: women having a multiple birth, teenagers (<18 years
of age), and low-risk women. A woman is classified as low risk if her birth
is a singleton, she is married, is aged 18 to 34 years, is having a second
or third live birth, and has at least 12 years of education. Women considered
to have high parity for their ages were also excluded from the low-risk group,
for example, an 18-year-old having her third birth.34
Twenty six percent of women having births in 1995 met this definition of low
The race categories are based on the self-reported race of the mother
and include white, African American, American Indian, and Asian or Pacific
Islander. Data for Hispanic women were not identifiable for many states during
the early part of the study period; therefore, only limited data for 1995
are included in this analysis. We compared women born in the United States
(the 50 states and the District of Columbia) with women born elsewhere.
We will present the trends in prenatal care utilization for the complete
population of US births for each of the 4 indices and for low-risk women for
the years 1981 to 1995. For women with multiple births and teenagers, we compare
utilization for the years 1981 and 1995.
Findings are based on essentially the complete population of US births
and are not subject to sampling error. Therefore, SEs or other sample statistics
are not presented for point estimates in Table 2, Table 3, and Table 4 or in Figure 1. 35
Multiple logistic regression was used to analyze the risk characteristics
of those women who had intensive prenatal care utilization according to the
R-GINDEX and APNCU Index in the years 1981 and 1995. We weighted the logistic
models by plurality status to account for the difference between the number
of births and the number of women giving birth. The parameters in the logistic
model were estimated by the maximum likelihood method. Adjusted odds ratios
and 95% confidence intervals were calculated from the logistic analyses.
A comparison of the national trends among the 4 indices produced strikingly
different results (Table 2). As
indicated above, the level of adequate prenatal care utilization, as measured
by the IOM Index and the trimester that care began, remained essentially unchanged
through the 1980s and only began to rise slightly in the 1990s.24
By contrast, the R-GINDEX and the APNCU Index, which use the full ACOG recommendations,
showed a steadily increasing trend toward more prenatal care utilization throughout
the study period, especially intensive utilization. While the percentage of
women having adequate use according to the IOM Index remained essentially
unchanged between 1981 and 1989 (67.0%-67.9%), the percentage of women having
either intensive or adequate use according to the R-GINDEX increased substantially,
from 32.7% to 40.0% in 1989, and continued to increase to 47.1% by 1995. Moreover,
the APNCU Index indicates that most of the change was toward more intensive
utilization; the percentage of women having intensive use increased from 18.4%
in 1981 to 28.8% in 1995, while the percentage of women with adequate use
remained essentially unchanged (45.1%-43.9%). All 4 indices, however, show
a relatively similar decrease in intermediate and inadequate use.
We also compared the trends in the 4 measures for the 3 risk groups.
Table 3 compares the 4 prenatal care measures
among singleton and multiple births in 1981 and 1995. The measures that include
an intensive use category indicate a dramatic change in the care of mothers
of multiples over this period. According to the R-GINDEX, while the percentage
of women with intensive use who had a singleton birth increased from 3.3%
to 6.3%, the percent of women who had a multiple birth receiving intensive
utilization jumped from 8.5% in 1981 to 22.8% in 1995. A similar increase
was noted for the APNCU Index: whereas about 47% of women with multiple births
had intensive use in 1981, more than 70% received intensive utilization by
1995, compared with only a 10 percentage point increase for singleton births.
By contrast, both the IOM Index and the trimester that care began indicate
that women with multiple births were only slightly more likely than women
with a singleton birth to either have adequate use or begin care in the first
trimester in both 1981 and 1995.
Table 4 reveals a very different
pattern of utilization by teenaged mothers over this period, as defined by
the various measures. Both the IOM Index and first trimester care use revealed
large disparities of inadequate utilization between teenagers and adults in
1981 and 1995, although the difference had narrowed somewhat by 1995, due
to major increases in adequate use by teenagers. On the other hand, adults
were only slightly more likely to receive intensive utilization than teenagers
as measured by both the R-GINDEX and the APNCU Index.
All indices show an increase in either intensive or adequate utilization
among low-risk women (Figure 1).
Low-risk women were more likely to begin care earlier than the overall population;
by 1995, 90.0% of low-risk women began care in the first trimester compared
with 81.6% in the overall population (Table
2). A similar pattern was evident for the IOM Index. Intensive use
also increased substantially for this population, with the yearly increases
for the APNCU Index and R-GINDEX mirroring the trends in the overall population.
Due to the substantial rise in the percentage of women receiving intensive
utilization, we examined the characteristics associated with intensive use
for the R-GINDEX and APNCU Index in 1981 and 1995 (Table 5). Factors associated with more intensive utilization according
to both the R-GINDEX and APNCU Index in 1981 were being a multiple birth,
being primiparous, being married, being of older maternal age (≥35 years),
and being of African American race. Conversely, women having at least a fourth
birth, teenagers, and Native American and Asian women were less likely to
have intensive use. By 1995, factors associated with intensive prenatal care
use based on the R-GINDEX and APNCU Index were somewhat different. Women with
multiple births were more likely to have intensive utilization in 1995 than
1981 (odds ratio [OR], 2.68 in 1981 and 3.82 in 1995 for the R-GINDEX; OR,
2.04 in 1981 and 2.73 in 1995 for the APNCU Index). Additionally, there was
a trend toward more intensive use among women with greater education. Among
the groups less likely to have intensive use in 1995 were women born outside
the United States (OR, 0.76 for the R-GINDEX; OR, 0.80 for the APNCU Index).
In 1981, there were no differences between US-born women and women born elsewhere
according to the R-GINDEX, while women born outside the United States were
more likely to receive intensive use on the APNCU Index. Although married
women were still more likely to have intensive utilization, there was a decline
between 1981 and 1995 (OR, 1.31 in 1981 and 1.16 in 1995 for the R-GINDEX;
OR, 1.62 in 1981 and 1.27 in 1995 for the APNCU Index).
We also examined factors associated with intensive prenatal use in 1995
creating a separate category for Hispanics. This did not alter the other factors
associated with intensive care. Hispanic women were less likely than non-Hispanic
white women to have intensive utilization according to the APNCU Index (OR,
0.80). (Data available on request from the senior author.)
This study of all births in the United States from 1981 to 1995 revealed
a major increase in prenatal care utilization, especially in intensive prenatal
care use, when using the newer indices of prenatal care utilization compared
with the traditional but less sensitive measures, which had not revealed this
trend. The traditional measures of prenatal care (the trimester that care
began and the IOM Index) suggested a basically stable pattern of early and
adequate use of prenatal care from 1981 to 1995. The flaws of these 2 measures
limited their sensitivity to the increasing visits.26
This increase in intensive utilization of prenatal care indicates a noteworthy
change in prenatal care practice trends in the United States.
The factors that underlie the observed pattern of change in intensive
use are open to speculation. During the early 1980s, perinatology (maternal-fetal
medicine) developed as a specialty and several obstetric diagnostic technologies
came into widespread use, eg, ultrasonography.36
Results from these diagnostic procedures may have led to an increase in the
number of prenatal care visits and intensive utilization.37
Escalating litigation in the obstetric field may have resulted in more cautious
practice patterns manifested by added referrals to the growing number of available
although others found no increase in the use of prenatal resources for low-risk
women by physicians with greater malpractice claims exposure.42
The ongoing expansion of Medicaid eligibility for pregnant women, which improved
access to and funding for prenatal care, may have also contributed to the
rising trend in intensive use in the 1990s.
The increase in intensive use was seen across all subpopulations examined
and suggests it is a universal phenomenon. The increase in intensive prenatal
care use by low-risk women in the United States suggests that the Public Health
Service's recommendations for fewer visits to low-risk women have had little
impact on prenatal care practice.21 The corresponding
rise in intensive utilization among women with some college education, typically
a group with fewer high-risk characteristics, further suggests that women
with potentially greater resources are receiving additional visits, regardless
of risk status.
Finally, the disproportionate growth in intensive utilization by women
with multiple births may stem from factors that underlie the increase in multiple
births in the United States, eg, increasing age of mothers and infertility
interventions,43,44 and possibly
a practice trend toward more aggressive management of these high-risk pregnancies.
The decline in intensive use of prenatal care by women born outside
the United States is counter to the overall trend and may reflect changes
in access to care. Ongoing monitoring of prenatal care use by these women
is indicated, given recent provisions in the welfare legislation that may
erode access to prenatal care.45
However, regardless of the causes of this trend, the health care cost
implications of this increase in intensive use must be explored. Although
the previously presumed cost benefit of prenatal care has been recently called
into question,46 it is unclear what cost implications
this increase in intensive use of care entails, as cost studies on prenatal
care have not specifically focused on intensive use nor have they considered
the potentially distinctive needs of this population.47,48
During this same period of increased utilization, the rates of low birth
weight and preterm birth in the United States have worsened.23
Although these prenatal care indices are measures of the utilization, and
not the content or appropriateness of care, the results suggest that simply
offering more prenatal care services without careful evaluation of the clinical
significance of the services provided may not lead to improved birth outcomes.
While both the R-GINDEX and the APNCU Index reveal a similar rise in
the intensive use of prenatal care, the difference in the proportion of women
each index places into this category is striking; eg, in 1995, the intensive
use percentage was 6.7 by the R-GINDEX and 28.8 by the APNCU Index. Previous
comparisons of these indices have stressed that differences in the conceptualization
and coding of intensive use by these indices underlie their disparate measures.27 It should be noted that a generally accepted standard
definition for intensive use does not exist.
The accuracy and completeness of reporting are potential limitations
to the prenatal care trends noted here.49-51
Other research has indicated that reporting of gestational age on the birth
certificate may not be accurate below 37 weeks, which can affect the accurate
classification of prenatal care use.52 We did
not see any differences in trends when we examined prenatal care utilization
among births less than 37 weeks (preterm) and those of 37 weeks or more. Other
studies have compared the agreement between prenatal care information on birth
certificates and medical records with mixed results. While some have not found
a high level of agreement,53 others have,54,55 particularly for women having adequate
Changes in reporting areas may also have affected the results.35 Two states did not collect information on the number
of prenatal visits during the earlier part of the study period. These states
were excluded from the analysis for the years where they had missing data.
For the years 1989 to 1995, we examined the trends in prenatal care use both
including and excluding these states and found negligible differences. Therefore,
we included all states for 1989 to 1995. Three states did not report education
during earlier parts of the study period. We examined the trends in prenatal
care utilization among low-risk women both including and excluding those states
in the years when there was full reporting and found few differences. We also
examined the risk of intensive utilization in 1995 both including and excluding
the 4 states that did not collect information on either prenatal visits or
education in 1981 and found no differences in the characteristics associated
with intensive use (data available on request from the authors).
Moreover, although the definition of a prenatal visit has remained constant,
it is possible that the increase in diagnostic procedures may have led to
some of these visits being counted inappropriately as prenatal visits, and
thus inaccurately inflating the number of visits for some women. However,
between 1990 and 1995, levels of intensive use for the R-GINDEX and the APNCU
Index increased among both women reported to have received or not received
an ultrasonogram (data available from the authors).
The newer measures of prenatal care utilization indicate that there
has been an increase in the number of visits that women receive that exceed
recommendations by ACOG. While there has been success in increasing prenatal
care utilization to high-risk women, the benefits of increasing obstetric
care to low-risk women are less immediately apparent. Given that the rates
of low birth weight and preterm birth in the United States have not improved
over this same period, investigations of cost benefit should explore the varied
short- and long-term outcomes that may be influenced by prenatal care use
to more completely evaluate the impact of these trends.57
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