Adams MM, Elam-Evans LD, Wilson HG, Gilbertz DA. Rates of and Factors Associated With Recurrence of Preterm Delivery. JAMA. 2000;283(12):1591-1596. doi:10.1001/jama.283.12.1591
Author Affiliations: World Health Organization Collaborating Center in Perinatal Care and Health Services Research in Maternal Child Health, Division of Reproductive Health, Centers for Disease Control and Prevention, Atlanta, Ga.
Context Information about risk of recurrent preterm delivery is useful to clinicians,
researchers, and policy makers for counseling, generating etiologic leads,
and measuring the related public health burden.
Objectives To identify the rate of recurrence of preterm delivery in second pregnancies,
factors associated with recurrence, and the percentage of preterm deliveries
in women with a history of preterm delivery.
Design and Setting Population-based cohort study of data from birth and fetal death certificates
from the state of Georgia between 1980 and 1995.
Subjects A total of 122,722 white and 56,174 black women with first and second
singleton deliveries at 20 to 44 weeks' gestation.
Main Outcome Measure Length of gestation (categorized as 20-31, 32-36, or ≥37 weeks) at
second delivery compared with length of gestation at first delivery, by age
Results Most women whose first delivery was preterm subsequently had term deliveries.
Of 1023 white women whose first delivery occurred at 20 to 31 weeks, 8.2%
(95% confidence interval [CI], 6.6%-10.1%) delivered their second birth at
20 to 31 weeks and 20.1% (95% CI, 17.7%-22.8%) at 32 to 36 weeks. Of 1084
comparable black women, 13.4% (95% CI, 11.4%-15.6%) delivered at 20 to 31
weeks and 23.4% (95% CI, 20.9%-26.1%) delivered at 32 to 36 weeks. Among women
whose first delivery occurred at 32 to 36 weeks, all corresponding rates were
lower than those whose first birth was at 20 to 31 weeks; the rates of second
birth at 20 to 31 weeks were substantially lower (for white women, 1.9% [95%
CI, 1.7%-2.2%]; for black women, 3.8% [95% CI, 3.4%-4.2%]). Compared with
women aged 20 to 49 years at their second delivery, women younger than 18
years had twice the risk of recurrence of delivery at 20 to 31 weeks. Of all
second deliveries at 20 to 31 weeks, 29.4% for white women and 37.8% for black
women were preceded by a preterm delivery.
Conclusions Our data suggest that recurrence of preterm delivery contributes a notable
portion of all preterm deliveries, especially at the shortest gestations.
In 1997 in the United States, approximately 10% (n = 310,843) of white
newborns and 18% (n = 104,152) of black newborns were delivered preterm, before
37 completed weeks of gestation.1 Preterm birth
increases the risks for infant morbidity and mortality. Clinicians can use
information on rates of repeat preterm delivery in counseling patients; those
who have already experienced a preterm delivery are likely to be especially
anxious for guidance. Researchers may find that identifying risk factors for
recurrent preterm delivery suggests hypotheses about the etiology of preterm
delivery, the cause of which is largely unknown.2
In addition, policy makers responsible for directing prenatal care and for
allocating research funds need to know the contribution of recurrent preterm
delivery to the overall rate of preterm delivery. We used birth and fetal
death certificates filed in Georgia from 1980 through 1995 to compute the
rate of recurrence of singleton preterm delivery in second pregnancies.
We used fetal death and birth certificates to identify successive pregnancies
occurring to individual mothers (pregnancy histories). Methods for constructing
pregnancy histories by linking vital records and an evaluation of these histories
have been published.3,4 From these
histories, we identified women whose first and second pregnancies ended in
singleton stillbirths or live births of newborns weighing at least 500 g or,
if birth weight was unknown, women who delivered at 20 weeks or more of gestation.
To ensure that we analyzed consecutive births, we required that the month
and year of the preceding birth listed on the second birth certificate match
exactly the month and year recorded on the certificate for the first birth.
We considered the length of gestation to be the number of full weeks between
the last menstrual period and birth, and we assessed the plausibility of this
variable using sex- and race-specific standards.5
We excluded those who had 1 or more pregnancies whose gestational length was
unknown, was shorter than 20 weeks or longer than 44 weeks, or was implausible
given the infant's birth weight. We restricted the study population to women
who were white or black.
All data for this study derived from fetal death or live birth certificates.
The outcome of interest was length of gestation in the second pregnancy, but
this measure was nonrandomly missing for a substantial percentage of pregnancies.
As birth weight was available for nearly all newborns, we considered it a
proxy for gestation and conducted parallel analyses using birth weight. We
analyzed all preterm deliveries (<37 weeks) as well as 3 subsets of these
deliveries composed of very preterm (20-27 and 28-31 weeks) and moderately
preterm (32-36 weeks) deliveries. These 3 subsets correspond to different
levels of risk for infant mortality and morbidity.6,7
Because the number of deliveries at 20 to 27 weeks' gestation was small, we
combined them in multivariate analyses with those at 28 to 31 weeks. For low
birth weight, we used the conventional subsets of very low birth weight (VLBW,
<1500 g) and moderately low birth weight (MLBW, 1500-2499 g).
Key variables were length of gestation and birth weight in the first
pregnancy (categorized in the same way as for the second pregnancy). In analyzing
the risk of recurrence of preterm delivery or delivery of low-birth-weight
newborns, we examined the following second-pregnancy variables that might
affect this risk: sex of the infant, interpregnancy interval, year of delivery,
presence or absence of the father's name on the birth certificate, maternal
education and smoking, and outcome (live or stillborn). We also checked for
the potentially confounding effects of these variables.
Interpregnancy interval was defined as the number of months between
the date of birth for the first pregnancy and the last menstrual period for
the second pregnancy. The father's name was coded as present for both pregnancies
or absent for 1 or both pregnancies. In Georgia during the study years, the
father's name was routinely listed on birth certificates of married women.
To be listed on the certificate of an unmarried woman, the father had to give
his written permission. Thus, women for whom the father's name was listed
for both pregnancies might, on average, have had greater paternal support.
Because information on maternal smoking was available only after 1988, analyses
that included smoking were restricted to second deliveries after that year.
All variables were categorical.
To evaluate potential bias in our analysis of preterm recurrence that
might be related to the availability of gestational-length data, we compared
the birth-weight distribution and personal characteristics of the group of
women for whom gestational-length data were available for both pregnancies
with those of women for whom gestational-length data were missing for 1 or
both pregnancies. We then computed the crude rates of preterm delivery for
the first and second pregnancy and rates of recurrence of preterm delivery.
Among the subsets of women whose first delivery was preterm or whose
first newborn had a low birth weight, we used logistic regression to look
for factors associated with recurrence of the same outcome of the first delivery.
For analyses of the preterm subsets, the comparison group consisted of women
who had term deliveries (≥37 weeks) in their second pregnancies. For analyses
of the low-birth-weight subsets, we used normal birth weight (≥2500 g)
as the control group. Because of racial differences in the background rates
of preterm delivery, we stratified all analyses by maternal race. We defined
confounding to be present when the adjusted odds ratio (OR) differed from
the crude OR by at least 10%. We assessed each model by using the Hosmer-Lemeshow
goodness-of-fit test and comparing the predicted and observed distributions
of outcomes. Because recurrences of preterm delivery or low-birth-weight newborns
were not rare, ORs computed from logistic regression were expected to overestimate
relative risks (RRs).8 When we used the method
of Zhang and Yu8 to compute the RRs from the
adjusted ORs, however, we found that the ORs only marginally overestimated
the RRs, presumably because nearly all the ORs were moderate. Hence, except
for a few illustrative comparisons of ORs and RRs, we present ORs only. We
excluded women from logistic regression models when information for at least
1 of the variables in the model was missing. Finally, we computed the percentage
of preterm deliveries in the second pregnancy that were preceded by a preterm
Analysis of these data was approved by the institutional review board
at the Centers for Disease Control and Prevention.
From 1980 through 1995, 1,820,110 live birth or fetal death events were
recorded in Georgia. We identified 155,519 white women and 75,556 black women
who had first and second singleton pregnancies during this period. Fifty-three
percent of white women and 55% of black women with a first delivery in 1980
through 1984 were included in these groups; for women with first deliveries
in 1985 through 1989 or after 1989, the comparable percentages were 48% and
52% for white women and 23% and 25% for black women, respectively.
Acceptable data for determining length of gestation for both the first
and second pregnancies were available for 122,722 (78.9%) of white women and
56,174 (74.3%) of black women. In contrast, newborns' birth-weight data for
both pregnancies were available for 98.6% of white women and 99.0% of black
women. Higher percentages of women who, in their first pregnancies, delivered
newborns weighing at least 2500 g had acceptable gestational-length data for
both pregnancies (white women, 80.4%; black women, 76.3%) than did women who
delivered newborns weighing 1500 to 2499 g (white women, 58.3%; black women,
61.3%) or less than 1500 g (white women, 63.7%; black women, 60.6%). Similar
patterns were observed for second pregnancies (data not shown). Also, among
both white women and black women, the subgroups of women who had characteristics
typically associated with increased risk for adverse pregnancy outcomes (eg,
they were young, unmarried, had a lower level of education, initiated prenatal
care late in the pregnancy, or had no prenatal care) had a lower percentage
of acceptable gestational-length data for both pregnancies than did women
at lower risk. Finally, for women of both races, the percentage with acceptable
data for gestational length of both pregnancies was stable across the study
years (data not shown).
For white women, rates of preterm delivery (<37 weeks), moderately
preterm delivery (32-36 weeks), very preterm delivery (20-31 weeks), low-birth-weight
newborns (<2500 g), MLBW newborns (1500-2499 g), and VLBW newborns (<1500
g) were higher for first pregnancies than for second pregnancies (Table 1). For black women, rates of preterm
delivery, moderately preterm delivery, low-birth-weight newborns, and MLBW
newborns decreased from the first to second pregnancies. In contrast, among
black women the rates of very preterm delivery and VLBW newborns were nearly
the same for the first and second pregnancies. Over time for both races, the
rate of very preterm delivery increased for first births but decreased among
second births (data not shown). Similar temporal trends occurred for the rate
of VLBW newborns.
Crude rates of recurrence of preterm delivery were higher for black
women than for white women. Among those in the study whose first pregnancy
ended in a preterm delivery, 19.9% of white women and 26.0% of black women
had a preterm delivery in their second pregnancy (data not shown). For both
white women and black women, the rate of preterm delivery in the second pregnancy
increased as the length of first pregnancy decreased (Figure 1). Among women whose first pregnancies lasted only 20 to
27 weeks, 28.9% of white women and 36.8% of black women had a preterm delivery
in their second pregnancy.
For both white women and black women whose first deliveries were very
preterm (20-31 weeks), the rate of preterm delivery in the second pregnancy
showed no sustained trend during the years of the study (Table 2). For example, among white women in this group, rates of
preterm second births were 24.2% for 1980 through 1984; 29.9% for 1985 through
1989; and 28.6% for 1990 through 1995. In contrast, over time among both white
women and black women who had very preterm deliveries in their first pregnancies,
very preterm deliveries made up a steadily declining percentage of all the
preterm deliveries in the second pregnancy. Among white women, this percentage
decreased from 36.8% in 1980 through 1984 to 23.8% in 1990 through 1995. For
comparable black women, this value was 53.4% in 1980 through 1984 and 31.2%
in 1990 through 1995. Similar patterns were not seen among women whose first
newborn had VLBW (Table 2).
For women of both races, the rate of preterm delivery or low-birth-weight
newborns in the second pregnancy among women whose first newborn was delivered
moderately preterm (32-36 weeks) or had MLBW (1500-2499 g) was lower in 1990
through 1995 than in 1980 through 1984. Additionally, very preterm or VLBW
births decreased as a percentage of all preterm or low-birth-weight births
Recurrence was defined for these analyses as a repeat delivery in the
same subset (eg, a very preterm delivery in the second pregnancy among women
whose first delivery was also very preterm). Maternal age was the only variable
we examined that was associated with recurrence in more than 1 analysis (Table 3). Among women whose first delivery
was very preterm or whose first newborn had VLBW, we had 1 statistically significant
finding. Specifically, black women whose first delivery was very preterm and
who were younger than 18 years at their second delivery were significantly
more likely than the referent group (women aged 20-49 years at their second
delivery) to have a very preterm delivery in the second pregnancy (adjusted
OR, 2.0; 95% CI, 1.2-3.5). The RR, which we computed from the adjusted OR,
was 1.8. Among white women, the adjusted OR was 2.3 (RR, 2.1) in the comparable
analysis, but this statistic was not significant (95% CI, 0.9-5.6). Among
black women whose first delivery was very preterm and who were younger than
18 years at the second pregnancy, 22.6% of the second deliveries were very
preterm and 26.0% were moderately preterm. Among comparable black women aged
20 to 49 years at the second birth, the rates were 10.8% and 22.6%, respectively.
Comparable values for white women were 15.1% and 26.4% (aged <18 years)
and 7.7% and 19.2% (aged 20-49 years). Among women whose first delivery was
very preterm, 5.2% of white women and 16.4% of black women were younger than
18 years at their second delivery.
Among the groups whose first delivery was at 32 to 36 weeks and those
whose first newborn had MLBW, 8 findings were significant (6 showing increased
risk and 2 showing decreased risk). Three significant adjusted ORs were for
maternal age younger than 18 years (20-49 years was the referent group). Black
women younger than 18 years whose first delivery was at 32 to 36 weeks had
an increased risk of recurrence for delivery at 32 to 36 weeks. Likewise,
black women younger than 18 years whose first newborn had MLBW had an increased
risk for recurrence of newborns with MLBW. Among white women in the same age
group, only those whose first newborn had MLBW were at increased risk for
recurrence. A second set of 4 findings was for interpregnancy interval; 12
to 47 months was the referent. Black women whose first delivery was at 32
to 36 weeks and who had intervals between pregnancies of more than 47 months
had a decreased risk of recurrent moderately preterm delivery. White women
whose first newborn had MLBW and had an interval between pregnancies of more
than 47 months had a moderately increased risk for recurrence of newborns
with MLBW. Black women whose first newborn had MLBW and had intervals of less
than 6 months or 6 to 11 months also had moderately increased risks for recurrence
of newborns with MLBW. Smoking during pregnancy was associated with reduced
risk of recurrence among white women whose first newborn had MLBW. Finally,
the goodness-of-fit of most of the models was reasonable.
Among preterm deliveries in the second pregnancy, 19.9% of those for
white women and 27.6% of those for black women were preceded by a preterm
delivery. In contrast, among women whose second pregnancy went to term, 6.3%
of white women and 13.7% of black women had a preterm delivery in their first
pregnancy. Furthermore, the percentage of second preterm deliveries preceded
by a preterm delivery increased as the gestation of the second pregnancy decreased.
Among women whose second delivery occurred at 20 to 31 weeks, 29.4% of white
women and 37.8% of black women had a preterm delivery in their first pregnancy.
For second deliveries at 32 to 36 weeks, 19.9% of those among white women
and 25.9% among black women were preceded by a preterm delivery.
For women whose first and second deliveries resulted in singleton births,
our results suggest several conclusions. Although a substantial percentage
of women whose first delivery is preterm will have a second preterm delivery,
most will subsequently have a term delivery. At least in Georgia, the rate
of preterm delivery for second pregnancies among women whose first pregnancy
was very preterm did not change from 1980 through 1995. In recent years, however,
more recurrent preterm deliveries are occurring closer to term, which would
reduce risks of death and severe disability. In contrast, for women who delivered
their first newborns at 32 to 36 weeks, the crude rate of recurrent preterm
delivery declined moderately. Also, being younger than 18 years at the second
delivery may increase the odds of repeating very preterm deliveries and of
having VLBW newborns. Finally, first preterm deliveries are relatively common
among women whose second deliveries are very preterm (29% for white women
and 38% for black women).
Many of the strengths and weaknesses of our findings relate to the vital
records data from which they derive. The population-based nature of vital
records reduced the potential selection bias that is often a concern with
clinically based data. Furthermore, the large number of women studied permitted
analysis of numerous subgroups; of note are our data analyses for black women,
who generally have a high rate of preterm delivery. Conversely, clinically
based data, which typically include many fewer women, offer a richness of
detail and level of accuracy that are not available from vital records. For
example, vital records do not permit a distinction between spontaneous and
medically indicated preterm births, an important difference because risk of
recurrence has been shown to vary between these categories.9
In addition, legitimate concerns have been raised about the accuracy of data
about gestational length on birth certificates.10,11
By editing the data to exclude implausible gestational lengths, we may have
introduced bias because we excluded a greater proportion of women who delivered
low-birth-weight newborns than women who delivered normal-birth-weight newborns.
That most of our parallel analyses of birth weight had findings similar to
our length-of-gestation analyses, however, lends credibility to the analyses
based on length of gestation.
Because we only had data for births in Georgia from 1980 through 1995,
we necessarily excluded women whose first or second pregnancy occurred before
or after the study period or in another state. We do not know how these exclusions
and omissions may have influenced the findings. Evaluation of the pregnancy
histories in the database, however, showed them to be complete.4
Our results confirm past observations that women whose first pregnancies
resulted in preterm deliveries have increased risks of preterm deliveries
in their second pregnancies.12,13
Our results also confirm that teenagers, especially young black teenagers,
have a high rate of recurrence of preterm delivery.14
Finally, our results also show a relationship between degree of preterm delivery
in the first pregnancy and likelihood of preterm delivery in the second.15- 17 By examining the
gestational-length distribution in the second pregnancy, we show that women
whose first newborns were delivered at 20 to 31 weeks have increased risks
for preterm delivery in the second birth, especially for delivery at 20 to
31 weeks. The risk of recurrent preterm delivery appears not to have changed
during the study years, despite the increase in preterm delivery that has
been observed in the United States and Canada.1,18,19
Recent reports suggest that short cervical length, the detection of
fetal fibronectin, and bacterial vaginosis during pregnancy increase the risk
of spontaneous preterm delivery.20- 22
Because cervical length is a constitutional factor that persists within an
individual, women with a short cervix likely face a high risk of preterm delivery
in all their pregnancies. In contrast, the other 2 risk factors may not have
the same persistence. Other promising findings suggest that interventions
during pregnancy can reduce the recurrence of preterm delivery.2
Although women who have experienced 1 singleton preterm delivery clearly have
increased absolute risks and RRs for a second singleton preterm birth, most
of these women will deliver their next newborns at term. Young teenagers may
prove an exception, at least among those whose first delivery was very preterm
(20-31 weeks). In this group, we found that among those younger than 18 years
at their second delivery, 41.5% of white teenagers and 48.6% of black teenagers
The etiology of preterm delivery remains elusive.2
Whether recurrent preterm deliveries share the same etiology as incident preterm
deliveries is unknown. Because a substantial percentage of preterm deliveries
in second pregnancies are associated with recurrence, additional research
is needed to identify the causes of recurrence.