Background Pregnancy in adolescence is associated with an excessrisk of poor outcomes, including low birth weight and prematurity.Whether this association simply reflects the deleterious sociodemographicenvironment of most pregnant teenagers or whether biologic immaturityis also causally implicated is not known.
Methods To determine whether a young age confers an intrinsicrisk of adverse outcomes of pregnancy, we performed stratifiedanalyses of 134,088 white girls and women, 13 to 24 years old,in Utah who delivered singleton, first-born children between1970 and 1990. Relative risk for subgroups of this study populationwas examined to eliminate the confounding influence of maritalstatus, educational level, and the adequacy of prenatal care.The adjusted relative risk for the entire study group was calculatedas the weighted average of the stratum-specific risks.
Results Among white married mothers with educational levelsappropriate for their ages who received adequate prenatal care,younger teenage mothers (13 to 17 years of age) had a significantlyhigher risk (P<0.001) than mothers who were 20 to 24 yearsof age of delivering an infant who had low birth weight (relativerisk, 1.7; 95 percent confidence interval, 1.5 to 2.0), whowas delivered prematurely (relative risk, 1.9; 95 percent confidenceinterval, 1.7 to 2.1), or who was small for gestational age(relative risk, 1.3; 95 percent confidence interval, 1.2 to1.4). Older teenage mothers (18 or 19 years of age) also hada significant increase in these risks. Even though sociodemographicvariables associated with teenage pregnancy increase the riskof adverse outcomes, the relative risk remained significantlyelevated for both younger and older teenage mothers after adjustmentfor marital status, level of education, and adequacy of prenatalcare.
Conclusions In a study of mothers 13 to 24 years old who hadthe characteristics of most white, middle-class Americans, ayounger age conferred an increased risk of adverse pregnancyoutcomes that was independent of important confounding sociodemographicfactors.
During the past decade in the United States, approximately 10percent of teenage girls from 15 to 19 years old became pregnant.1Births to mothers in this age group now account for approximately13 percent of all live births in the United States.2 This patternis a source of societal concern, since teenage mothers havean increased risk of having low-birth-weight babies, prematurebabies, and babies who die during the first year of life3,4;they thus have a disproportionate share of all adverse outcomesof pregnancy. Despite the magnitude of the problem, it is unknownwhether the poor outcomes of teenage pregnancy are partly attributableto the biologic fact of a young maternal age or are solely theconsequence of sociodemographic factors generally associatedwith pregnancy among teenagers. Teenage mothers are more likelythan older mothers to be nonwhite, poor, less well educated,and unmarried, and they are less likely to have received earlyprenatal care1,5 — all known risk factors for low birthweight in their babies.6,7 Hence, the biologic risk associatedwith a young maternal age may have been exaggerated in previousstudies because of inadequate control for sociodemographic riskfactors.8,9,10,11 Studies in animals and a small number of epidemiologicstudies suggest, however, that a young age alone may be an independentrisk factor for adverse outcomes of pregnancy.4,5,6,12
Vital-statistics data from Utah offer an excellent opportunityto evaluate the biologic risk associated with pregnancy in adolescents,since the mothers in this state are largely white and married,have generally received adequate prenatal care, and usuallyhave a healthy lifestyle. The rate of smoking among women 18years of age or older in Utah13 is half the national averageof 24 percent,14 and the use of alcohol and drugs is also lowerthan in the country as a whole.15,16 Among the states, Utahranks 47th in the percentage of children living below the povertylevel and 10th in the proportion of high-school graduates.17We undertook this study to assess whether, in a population thatis largely white and middle-class, a young maternal age is associatedwith a high intrinsic risk of adverse outcomes of pregnancy.
Methods
Study Subjects
Of a total of 762,893 babies born in Utah between 1970 and 1990,150,508 were white, singleton, first-born infants whose motherswere 13 to 24 years of age. Of these infants, 134,088 were selectedfor the study because their records contained complete dataon the following variables: birth weight, duration of gestation,mother's age, mother's marital status, mother's educationallevel, and prenatal care received. The study protocol was approvedby the local institutional review board.
Definition of Variables
Outcome Variables
Low birth weight was defined as a weight below 2500 g at birth,and prematurity as delivery at a gestational age of less than37 weeks.18 Small-for-gestational-age infants were defined asthose with birth weights below the 10th percentile for gestationalage and sex; an iterative method was used to compensate forthe misclassification of gestational age among babies born prematurely.19
Teenage mothers were divided into two age groups: those 13 to17 years old and those 18 to 19 years old. Since mothers from20 to 24 years old had the lowest risk of all three adverseoutcomes, they served as the reference group in all comparisons.
Covariates
The mothers were categorized as unmarried or married. Prenatalcare was categorized as adequate, intermediate, or inadequateaccording to the criteria of the National Institute of Medicine,20which are based on the trimester during which prenatal careis initiated and the total number of prenatal visits. A mother'seducational level was defined as appropriate or inappropriatefor her age, so that a consistent classification could be usedfor both younger and older mothers. Mothers older than 19 wereconsidered to have an age-appropriate educational level if theyhad completed high school, whereas younger mothers had to havecompleted the minimal number of grades for their age.
Statistical Analysis
To eliminate the potentially confounding effects of sociodemographiccovariates, deliveries were cross-classified according to themother's marital status, the adequacy of prenatal care, andthe mother's educational level, yielding direct estimates ofrelative risk in subgroups defined by these variables (stratum-specificrelative risks) and 95 percent confidence intervals.21 The effectsof sociodemographic covariates were successively eliminatedby restricting the comparisons to mothers who were characterizedas having received adequate prenatal care; as married; as marriedand having received adequate prenatal care; and as married withan age-appropriate level of education and having received adequateprenatal care. Since in this final category the potential influenceof a deleterious sociodemographic environment is minimized,the risk for the mothers in this category was assumed to reflectthe underlying biologic risk. Adjusted relative risks for theentire study group (population-wide relative risks) and 95 percentconfidence intervals were calculated by pooling the appropriatelyweighted logarithms of the individual stratum-specific risks.21Two-tailed tests were used to evaluate the significance of thedifference between the logarithms of the individual risks,21and interactions in the multiway tables were tested with program4f of the BMDP statistical package.22
Results
In the entire cohort of infants born in Utah from 1970 to 1990,170,699 were white, singleton, first-born infants with completedata. Of these, 53 percent were born to mothers who were 20to 24 years old, 9 percent were born to younger teenage mothers(those <17 years of age), and 17 percent were born to olderteenage mothers (those 18 or 19 years of age). The younger teenagemothers were the most likely of these three groups to be unmarriedand the least likely to have received adequate prenatal care,whereas the older teenage mothers were the most likely to havean age-inappropriate educational level (Table 1). Since thesesummary statistics reflect data on the mothers of babies bornover a 20-year period, we evaluated these distributions foreach year (data not shown). There was no temporal trend in thedistribution of categories of prenatal care for any age group.However, there was a small decline (<2 percent) in the proportionof mothers with age-appropriate education, and, in concert withnational trends, the proportion of births to unmarried mothersincreased approximately three-to-fourfold for all three agegroups during this period.
Table 1. Characteristics of the Mothers in the Study Population, According to Age.
The overall incidence of low birth weight was highest amongthe babies born to the younger teenage mothers (7 percent) andlowest among those whose mothers were 20 to 24 years old (4percent) (Table 2). Similarly, the younger teenage mothers hadthe highest percentage of premature infants (10 percent) andsmall-for-gestational-age infants (14 percent). In all age groups,the babies born to mothers who were unmarried, who had not receivedadequate prenatal care, or who had an age-inappropriate educationallevel had an increased risk of all three adverse outcomes (Table 2).Inadequate prenatal care was most strongly associated withadverse outcomes: within an age group, mothers who did not receiveadequate care were approximately twice as likely to have low-birth-weightbabies as those who received adequate care and more than twiceas likely to have premature babies.
Table 2. Frequency of Adverse Outcomes of Pregnancy, According to the Mother's Age and Selected Characteristics.
Since adolescent mothers are more likely than older mothersto have sociodemographic characteristics associated with adverseoutcomes of pregnancy, we eliminated the confounding effectsof marital status, educational level, and adequacy of prenatalcare by cross-classifying births according to these covariates.The distribution of births among mothers with age-appropriateeducational levels indicates that, for all three age groups,most births occur among mothers in the more favorable sociodemographiccategories (married and having received adequate prenatal care),and the fewest to mothers who did not receive adequate prenatalcare (Table 3). The inverse relation between maternal age andthe proportion of births in the least favorable categories emphasizesthe importance of including sociodemographic covariates in theanalysis.
Table 3. Number of Infants Born to Mothers with Age-Appropriate Educational Levels in Each Age Group, According to Marital Status and the Adequacy of Prenatal Care.
The estimated relative risks for different subgroups definedaccording to sociodemographic covariates, plus the adjustedpopulation-wide relative risks for both groups of teenage mothersas compared with mothers 20 to 24 years of age, are shown inTable 4. The relative risks in the subgroup with the most favorablesociodemographic characteristics (i.e., mothers who were marriedand had adequate prenatal care and age-appropriate educationallevel) represent estimates of the intrinsic biologic risk ofadverse outcomes associated with a young age. Teenage mothersin this sociodemographic category had a significant elevationin the risk of all three adverse outcomes; the greatest elevationwas in the risk of prematurity among the infants born to theyounger teenage mothers (relative risk, 1.9; P<0.001). Thebabies of younger teenage mothers also had an increased riskof low birth weight (relative risk, 1.7; P<0.001). Olderteenage mothers had lower, but still significant, risks of havingpremature infants or infants with low birth weights.
Table 4. Relative Risk of Low Birth Weight, Prematurity, and Small Size for Gestational Age among Infants Born to Younger and Older Teenage Mothers, as Compared with Mothers 20 to 24 Years of Age.
To determine whether temporal trends influenced these results,we repeated the analysis after dividing the mothers in the mostfavorable sociodemographic category into five-year age cohorts(data not shown). For younger teenage mothers, the risk of allthree adverse outcomes remained significantly elevated in eachcohort, and apart from a slight decline in the risk of low birthweight and prematurity, there was no significant temporal trend.Older teenage mothers in each age cohort also had significantlyhigher risks, though the decline in the risk of prematurityover time was more marked. We also evaluated the outcome amongvery young teenage mothers (those 13 to 15 years of age) todetermine whether the intrinsic risk of adverse outcomes ofpregnancy was inversely proportional to maternal age. Amongthe infants born to young girls who were married, had age-appropriateeducational levels, and had received adequate prenatal care,the risks of all three adverse outcomes were significantly higherthan the risks among the infants born to mothers 16 to 17 yearsold. The relative risk of low birth weight was 2.8 (95 percentconfidence interval, 1.9 to 4.0) and that of prematurity was2.6 (95 percent confidence interval, 1.9 to 3.6), whereas therelative risk of small size for gestational age was 1.4 (95percent confidence interval, 1.1 to 1.8).
Although statistically significant, the adjusted relative risksfor the entire study group shown in Table 4 were consistentlylower than the relative risks for the most favorable stratum.However, the estimated adjusted relative risk for the entiregroup is potentially misleading because the existence of significantthree-way interactions within the complete data set suggeststhat individual stratum-specific risks are likely to differsignificantly. For example, analysis of all the subgroups inwhich the mothers had age-appropriate educational levels identifiedconsiderable variation in the risk of all three outcomes (Table 5).For both older and younger teenage mothers in this group,the relative risk of low birth weight and prematurity in theirinfants was significantly higher among those who were marriedbut had not received adequate prenatal care than among thosewho were unmarried and had intermediate levels of care. Similarly,among teenage mothers who received intermediate prenatal care,the relative risk of having a small-for-gestational-age infantwas significantly higher for those who were married than forthose who were unmarried. The observation that teenage motherswith less-than-optimal sociodemographic characteristics tendto have the same risk as mothers from 20 to 24 years of age(i.e., relative risk close to 1.0) suggests that the intrinsicrisk due to young maternal age has its greatest effect whenthe sociodemographic environment is favorable.
Table 5. Relative Risk of Adverse Outcomes of Pregnancy among Mothers with Age-Appropriate Educational Levels.
Because information about smoking by mothers during pregnancywas available for births in Utah in 1989 and 1990, we stratifiedthe analyses according to smoking history. The magnitude ofthe relative risks of all three adverse outcomes among the infantsof teenage mothers who did not smoke during pregnancy (Table 6)was similar to that in the full analysis (Table 4), eventhough the small sample precluded statistical significance.Notably, nonsmoking younger teenage mothers had a significantlyhigher risk of delivering a low-birth-weight infant than nonsmokingmothers from 20 to 24 years of age (relative risk, 2.0; 95 percentconfidence interval, 1.2 to 3.1) and also a higher risk of deliveringpremature and small-for-gestational-age infants. Among the nonsmokingolder teenage mothers, the relative risk of delivering a small-for-gestational-ageinfant was higher than in the previous analysis, but the relativerisks of low birth weight and prematurity declined.
Table 6. Relative Risk of Low Birth Weight, Prematurity, and Small Size for Gestational Age among Infants Born in 1989 or 1990 to Nonsmoking Teenage Mothers Who Were Married, Had Age-Appropriate Educational Levels, and Received Adequate Prenatal Care.
Discussion
Our analysis confirms the influence of sociodemographic factorson reproductive outcome; inadequate prenatal care, in particular,was associated with a marked increase in prematurity. This findingis consistent with the results of many previous studies, indicatingthat sociodemographic factors and the adequacy of prenatal carehave important effects on the outcomes of pregnancy among teenagers.5,6,7,20,23,24Our results also indicate, however, that although teenage mothershave a significantly elevated risk of delivering low-birth-weight,premature, and small-for-gestational-age infants, these risksremain significant even when the analysis is limited to marriedmothers with age-appropriate educational levels who receiveadequate prenatal care. This elevation in risk, consistent overthe 20-year period we studied, suggests that a young age inthe mother intrinsically increases the risk of adverse outcomesof pregnancy. That the relative risks were highest among theyoungest mothers in the more favorable sociodemographic stratastrengthens the case that young age is an inherent risk factorand challenges the contention that teenage mothers who receiveadequate prenatal care will have reproductive outcomes as goodas, or better than, those of older mothers.10,23,25 Our datasuggest that adequate prenatal care does not completely eliminatethe risks inherent in teenage pregnancy, presumably becausebiologic immaturity increases the risk of a poor outcome.
Our estimates could have been influenced by the fact that wehad no data on psychosocial attributes that are believed toincrease the risk of adverse outcomes of pregnancy, such asemotional stress and lack of family support.26 However, anybias is likely to have been minimal, since our main analysiswas restricted to married teenage mothers, who are likely toreceive both emotional and economic support. Similarly, we couldnot control for the use of illicit drugs such as cocaine, whichare known to influence reproductive outcomes.27 However, datafrom a random sample of pregnant women in Utah indicate a rateof cocaine use of only 1 percent, and a rate of 8 percent forall illicit drugs and alcohol combined.28 Moreover, a population-basedsurvey indicates that teenagers in Utah have lower rates ofillicit-drug use than teenagers in the nation as a whole.16Therefore, it is unlikely that illicit-drug use by pregnantteenagers influenced our results.
Two general features of biologic immaturity could have a rolein increasing the risk of adverse outcomes: a young gynecologicage (defined as conception within two years after menarche)29and the effect of a girl's becoming pregnant before her owngrowth has ceased.30,31,32 Immaturity of the uterine or cervicalblood supply may predispose teenage mothers to subclinical infection,an increase in prostaglandin production, and a consequent increasein the incidence of preterm delivery. Teenage mothers who themselvescontinue to grow during pregnancy could compete with the developingfetus for nutrients, to the detriment of the fetus. This suppositionis supported by evidence that weight gain during pregnancy maybe more critical for teenage mothers than for older mothers.31,32Hence, the intrinsic increase in the risk of adverse outcomesof pregnancy among teenagers in Utah is likely to be partlyattributable to young gynecologic age or inadequate weight gain.
Since white mothers have 67 percent of the infants born to teenagersin the United States,33 we believe our results have importantimplications. Becoming pregnant as a young teenager can resultin an intrinsic increase in the risk of adverse outcomes ofpregnancy, quite apart from the increased risk due to the adversesocial and behavioral factors that are frequently associatedwith teenage pregnancy. Consequently, efforts to improve thesociodemographic environment of pregnant teenagers may reducetheir risk of poor reproductive outcomes but will not eliminateit. Our data support the notion that every effort should bemade to persuade pregnant teenagers to obtain adequate prenatalcare and to adopt a healthy lifestyle. However, our resultsalso suggest that the burden of teenage pregnancy will remainunacceptably high unless efforts are made to identify the intrinsicbiologic factors that contribute to the increased risk of adverseoutcomes of pregnancy among younger mothers and then to findways to minimize the effects of these intrinsic factors.
Supported by a grant (1R01 HD23492) from the National Institutesof Health and by a grant from the Willard L. Eccles Foundation.
We thank Pat Corneli for statistical advice.
Source Information
From the Department of Human Genetics, University of Utah (A.M.F., R.H.W.), and the Bureau of Vital Records, Utah Department of Health (J.E.B.) — both in Salt Lake City. Presented in part at the annual meeting of the American Association of Physical Anthropologists, Las Vegas, April 3, 1992.
Address reprint requests to Dr. Ward at the Department of Human Genetics, 2100 Eccles Institute of Human Genetics, University of Utah, Salt Lake City, UT 84112.
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