Predictors of Birth Weight and Gestational Age Among Adolescents

Abstract

Although pregnant adolescents are at high risk of poor birth outcomes, the majority of adolescents go on to have full-term, healthy babies. Data from the National Longitudinal Study of Adolescent Health, a longitudinal study of a nationally representative sample of adolescents in grades 7–12 in the United States who were surveyed from 1994–1995 through 2008, were used to examine the epidemiology of preterm birth and low birth weight within this population. Outcomes of pregnancies were reported by participants in the fourth wave of data collection (when participants were 24–32 years of age); data were compared between female participants who reported a first singleton livebirth at less than 20 years of age (n = 1,101) and those who were 20 years of age or older (n = 2,846). Multivariable modeling was used to model outcomes; predictors included demographic characteristics and maternal health and behavior. Among black adolescents, low parental educational levels and older age at pregnancy were associated with higher birth weight, whereas low parental educational levels and being on birth control when one got pregnant were associated with higher gestational age. In nonblack adolescents, lower body mass index was associated with lower birth weight, whereas being unmarried was associated with lower gestational age. Predictors of birth outcomes may differ by age group and social context.

In 2008, there were 434,758 live births to mothers who were 15–19 of age in the United States (1). Giving birth during adolescence (before age 20 years) is associated with a number of pregnancy complications, including infant death (2–4), stillbirth (due largely to preterm labor and delivery) (5), congenital anomalies (6), preterm birth (PTB), and low birth weight (LBW) (7). For instance, in 2005, 13.3% of births to girls less than 15 years of age resulted in offspring with LBW. The rate for 15–19-year-old girls was 10.0%, whereas the overall rate was for all women was 8.2%. Teens less than 15 years of age are at a particularly high risk, although such pregnancies are fairly rare (8–10).

Several risk factors for LBW/PTB are known for adults, including black race, smoking, history of LBW/PTB, low weight gain, short maternal stature (11), low socioeconomic status, low weight, and nulliparity (12, 13). Teenage mothers are more likely to be unmarried, to smoke, to gain a low amount of weight during pregnancy (14), and to get inadequate prenatal care, which has been strongly associated with preterm birth in this group (15). However, only a few studies have addressed the risk factors specifically within the teenage age group. In 1 clinic-based study, underweight teens were at higher risk of PTB and having offspring with LBW (16). In a study using national birth certificate data, infant mortality among teenagers was associated with not reporting the father on the birth certificate, as well as with alcohol use, tobacco use, and low weight gain (10). A study of Australian teenage mothers using data from a national surveillance system found that smoking was associated with increased rates of small-for-gestational-age offspring, very PTB, and stillbirth, as was residence in a very remote area. Nulliparity was associated with an increased risk of small-for-gestational-age offspring but a reduced risk of very PTB (17). Birth outcomes were reported to be worse in second pregnancies among German adolescents based on data collected by obstetricians within a single region of Germany (18).

It is reasonable to think that risk factors might differ between adolescents and adults. For instance, in adults, married women have better birth outcomes than do single women (19). However, single adolescents may be more likely to live with their parents, and the resources of this arrangement might outweigh any support provided by marriage. Married or cohabiting teenage parents who do not live with both parents are less likely to graduate high school than are those who live with 2 parents (20). One study found that exposure to violence was associated with PTB in adolescents but not adults (21). There is also evidence that racial disparities are not as large within teenage populations (22). Although young black teenagers are at particularly high risk of LBW (17.2% among girls <15 years of age) compared with national averages, there also has been some research indicating that black adolescents actually have better birth outcomes than do older black women (23).

The purpose of the present article is to further explore the behavioral and demographic determinants of birth outcomes among adolescents in a nationally representative sample and to explore whether predictors of birth outcomes among teens are different from those of women who first give birth later in life. This analysis will add to past American studies by utilizing a nationally representative sample that is more diverse than prior clinic-based studies but includes more personal-level information than birth certificate studies; by examining multiple risk factors simultaneously; and by focusing on both PTB and LBW. Such analyses are also needed to add to the international literature given the different socioeconomic, racial, and health services context of American teens versus their counterparts elsewhere.

MATERIALS AND METHODS

Data from waves I and IV of the National Longitudinal Study of Adolescent Health (Add Health) contractual data set were utilized. Add Health is a prospective cohort study of a nationally representative sample of young persons enrolled in grades 7–12 in the 1994–1995 school year (wave I) (24). Follow-up interviews were conducted in 1996 (wave II), 2001 (wave III), and 2007–2008 (wave IV). Add Health utilized a multistage probability clustered sampling design to obtain its original wave I sample. The first stage of sampling included a stratified random sample of all public and private high schools in the United States. A middle school with students who largely attended the selected high school was also recruited from each participating community. In-school surveys were attempted with all students attending participating schools; a total of 90,118 were completed. In the second wave I sampling stage, a sample of adolescents was drawn for in-depth in-home interviews, consisting of a random core sample plus selected special oversamples; a total of 20,745 interviews were conducted at this stage. At wave II, most students (except wave I seniors) were eligible for re-interview; at waves III and IV, all respondents to the wave I in-home interview were eligible for re-interview. A total of 15,701 interviews were conducted at wave IV (80.3% response rate). Sampling weights were adjusted for both unequal probabilities of selection into the original sample and for loss to follow-up.

The major goal of the analysis was to compare women who gave birth before 20 years of age with women who gave birth when they were 20 years of age or older. We applied a number of inclusion criteria for our analyses. First, we limited it to females who participated in wave IV, as that was the only wave by which all respondents had completed their teenage years and thus had complete data on teen births. Second, we limited our analysis to participants with valid sampling weights to make generalizations to the wider US population. Third, we limited our study to women whose first births occurred after wave I to ensure the temporal ordering of predictors and outcomes. Fourth, we limited analyses to first singleton livebirths. Finally, we limited the study to persons for whom we had complete information on outcomes, age, race, and smoking status during pregnancy (described below). This left us with an analysis sample of 1,101 teen births and 2,846 adult births. We also examined 2 somewhat more distinct groups, women who gave birth before 18 years of age (n = 335) and women who gave birth at 24 years of age or older (n = 1,394) (25) as a sensitivity analysis.

Measures

At wave IV, girls were asked about previous pregnancies and their outcomes. If they indicated they had given birth, they were asked, “How much did the baby weigh at birth?” “Was [baby's name] born before or after [his/her] due date?” and then “How many weeks or days early/late was [baby's name] born?” This was subtracted from 40 weeks to calculate gestational age. In this analysis, both continuous forms and indicators for LBW (<2,500 g) and PTB (before 37 weeks of gestation) were included.

Sociodemographic characteristics that have been related to birth outcomes among adults or adolescents in the prior literature (race, ethnicity, family structure, and parental educational level) were included in analyses. All such variables were measured at wave I. Age at wave I was included as a continuous variable to control for possible cohort differences. Self-reported race was specified as black/nonblack and ethnicity as Hispanic/not Hispanic. Family structure (2 biological parents vs. other) is an important predictor for many adolescent health and social outcomes (26–28). Parental educational level (higher of either co-residential mother or father: less than high school diploma, high school diploma/general equivalency diploma, some postsecondary, or college degree) was included as an indicator of family-of-origin socioeconomic status.

A number of behavioral and maternal health variables were also included based on their prior associations with adverse birth outcomes in either adult or adolescent populations. Prepregnancy maternal health variables included gravidity (1 vs. >1) and prepregnancy body mass index (BMI; weight (kg)/height (m)²) at wave I (underweight, normal weight, overweight, or obese) and at the wave of data collection before pregnancy. Pregnancy-specific behaviors measured at wave IV included smoking during pregnancy (yes or no), prenatal care use (yes or no), and trimester of prenatal care initiation (first trimester, second trimester, or third trimester or no prenatal care). Year of offspring's birth (1995–1999 vs. 2000–2008) was also included to control for possible period effects, given the large decline in teen pregnancy rates between the early 1990s and mid-2000s. Lastly, whether the participant was taking birth control when she became pregnant (yes or no) was used as a proxy for intent to get pregnant.

Statistical analyses

All analyses were conducted in SAS, version 9.2 (SAS Institute, Inc., Cary, North Carolina) using survey procedures, which apply population weights and adjust standard errors for nonindependence between observations due to school-based sampling. Analyses began with descriptive statistics of the analysis sample's demographic characteristics by first birth timing (teenaged mother vs. not). Next, bivariate relations between demographic and behavioral variables and birth outcomes (LBW, PTB, birth weight, and gestational age) were examined using χ² analyses or analysis of variance/t tests, depending on the specification of the predictor and outcome variables. Additionally, differences between teen and adult mothers in these associations were tested using interactions between birth timing (teenaged mother vs. not) and the predictor variable in logistic or linear models. In the final step, all predictors were entered into multivariable models to test their relation with the outcome, adjusted for other variables. Because of the known differences in the context of adolescent pregnancy among racial groups (29), interactions with race (black vs. nonblack) were examined using product terms, and models are presented stratified on this variable. Variables were omitted from these models when cell sizes were less than 5.

The Add Health Study was approved by the institutional review board of the University of North Carolina at Chapel Hill, and this analysis was approved by the institutional review board of Tulane University.

RESULTS

Characteristics of the 2 groups are provided in Table 1. Women who gave birth as adolescents (ages 13–19 years) were more likely to be black or Hispanic, to live in a household without 2 biological parents present, to have parents with less than a high school education, to have an unemployed parent, and to be unmarried than were women who gave birth at older ages (ages 20–33 years). They were more likely to smoke (26% vs. 19%, P < 0.01) and to be using birth control (30% vs. 20%, P < 0.01) when they got pregnant. They were not, however, at particularly high risk of PTB or LBW, and the mean gestational age was actually a bit longer in the adolescents.

Table 1.

Maternal Characteristics of Female Participants Reporting Singleton Livebirths in Wave IV of the National Longitudinal Study of Adolescent Health (n = 3,947), United States, 1996–2007

	Age at First Birth
Characteristic	<20 Years (n = 1,101)		≥20 Years (n = 2,846)		P Value
	No. of Participants	%a	No. of Participants	%a
Baseline age, yearsb	15.48 (0.14)		16.25 (0.11)		<0.01
Baseline BMIb,c	22.32 (0.17)		22.21 (0.14)		0.54
Proximal BMIb,c	22.95 (0.16)		24.32 (0.16)		<0.01
Birth weight, kgb	3.25 (0.02)		3.27 (0.02)		0.42
Gestational age, weeksb	39.27 (0.09)		39.04 (0.06)		0.03
Baseline BMI category					0.30
Underweight	128	13.61	376	14.76
Normal weight	704	66.37	1,823	65.89
Overweight	169	14.92	373	12.89
Obese	58	5.10	193	6.46
Proximal BMI category					<0.01
Underweight	112	10.64	223	7.83
Normal weight	722	66.08	1,686	59.89
Overweight	167	14.94	505	17.03
Obese	91	8.34	431	15.25
Race/ethnicity					<0.01
White	498	57.65	1,595	70.38
Black	344	23.79	596	15.25
Hispanic	200	15.01	462	9.83
Other	59	3.55	193	4.54
Gravidity					<0.01
Primigravid	935	86.83	2,189	78.77
Multigravid	166	13.17	657	21.23
Household structure					<0.01
2 biological parents at home	386	36.29	1,434	53.17
Other	715	63.71	1,403	49.83
Parental educational level					<0.01
High school graduate or higher	797	79.25	2,312	86.48
Less than high school graduate	226	20.75	396	13.52
Parental unemployment					<0.01
Yes	84	9.37	148	5.31
No	849	90.64	2,285	94.69
Smoking during pregnancy					<0.01
No	859	73.71	2,383	80.62
Yes	242	26.29	463	19.38
Received prenatal care					0.09
No	36	3.09	60	1.80
Yes	1,065	96.91	2,785	98.20
Initiation of prenatal care					<0.01
First trimester	772	69.88	2,388	85.55
Second trimester	217	21.64	297	10.43
Third trimester or none	91	8.48	128	4.02
On birth control when got pregnant					<0.01
No	783	69.76	2,257	80.15
Yes	316	30.24	586	19.85
Marital status					<0.01
Married	174	16.52	1,469	53.92
Cohabitating	286	26.11	725	25.32
Other	641	57.37	652	20.76
Low birth weight					0.72
No	1,011	91.47	2,593	90.93
Yes	90	8.53	253	9.07
Preterm birth					0.49
No	1,003	91.74	2,595	90.72
Yes	98	8.26	251	9.28

Abbreviation: BMI, body mass index.

^a All percentages weighted for sampling design.

^b Mean (standard error).

^c Weight (kg)/height (m)².

In bivariate analyses of high-risk outcomes (Table 2), predictors of LBW in the older women included higher baseline BMI, black race, being primigravid, and not receiving prenatal care. In the adolescents, black race, being primigravid, and not receiving prenatal care were associated with a higher risk of having had a baby with a LBW. Predictors of PTB (Table 3) among the adult mothers included Hispanic ethnicity, not receiving prenatal care, and initiating prenatal care after the first trimester, whereas for adolescent mothers, being married or cohabiting were the only characteristics associated with PTB. Results were similar for continuous outcomes for adults; mean birth weight was lower in adult smokers than nonsmokers (3.19 kg vs. 3.29 kg, P = 0.01) but not in adolescents (3.23 kg vs. 3.25 kg, P = 0.79) (Tables 4 and 5).

Table 2.

Bivariate Analysis of Maternal Characteristics and Low Birth Weight, by Maternal Age, in the National Longitudinal Study of Adolescent Health (n = 3,947), United States 1996–2007

Characteristic	Age at First Birth						P for Interaction
	<20 Years			≥20 Years
	LBW		P Value	LBW		P Value
	No. of Participants	%a		No. of Participants	%a
Baseline age, yearsb	15.38 (0.23)		0.66	15.95 (0.22)		0.05	0.56
Baseline BMIb,c	21.69 (0.46)		0.19	23.26 (0.40)		<0.01	0.01
Proximal BMIb,c	22.35 (0.63)		0.37	25.62 (0.53)		<0.01	0.08
Baseline BMI category			0.68			<0.01	0.05
Underweight	12	9.72		26	5.84
Normal weight	56	7.38		150	8.21
Overweight/obese	15	6.79		69	13.46
Proximal BMI category			0.10			0.01	0.03
Underweight	14	14.88		21	7.31
Normal weight	56	7.90		128	7.73
Overweight/obese	18	6.67		104	11.98
Race			0.08			<0.01	0.92
Nonblack	56	7.27		170	8.10
Black	34	12.59		83	14.44
Ethnicity			0.41			0.28	0.21
Non-Hispanic	76	8.89		217	8.83
Hispanic	14	6.49		36	11.26
Gravidity			0.02			<0.01	0.73
Primigravid	79	9.24		207	10.18
Multigravid	11	3.86		46	4.94
Household structure			0.52			0.60	0.80
2 biological parents in the home	37	9.51		129	9.50
Other	53	7.98		124	8.64
Parental educational level			0.36			0.53	0.68
High school graduate or higher	61	7.68		208	9.01
Less than high school graduate	22	10.46		38	10.37
Parental unemployment			0.17			0.21	0.75
Yes	73	9.03		212	9.43
No	6	4.66		8	5.83
Smoking during pregnancy			0.32			0.94	0.42
No	73	9.18		207	9.04
Yes	17	6.72		46	9.17
Received prenatal care			0.07			0.05	0.80
No	5	20.75		8	18.95
Yes	85	8.14		245	8.89
Initiation of prenatal care			0.64			0.25	0.30
First trimester	67	9.28		206	8.60
Second trimester	15	6.58		27	11.41
Third trimester	8	9.04		17	13.41
Conceived while on birth control			0.62			0.78	0.57
No	65	8.20		202	9.18
Yes	25	9.33		51	8.70
Marital status (married vs. unmarried)			0.90			0.15	0.48
Unmarried	75	8.48		140	10.19
Married	15	8.81		113	8.11
Marital status			0.74			0.28	0.83
Married	15	8.81		113	8.11
Cohabitating	28	9.78		81	10.87
Other	47	7.89		59	9.36

Abbreviations: BMI, body mass index; LBW, low birth weight.

^a All percentages weighted for sampling design.

^b Mean (standard error).

^c Weight (kg)/height (m)².

Table 3.

Bivariate Analysis of Maternal Characteristics and Preterm Birth, by Maternal Age, in the National Longitudinal Study of Adolescent Health (n = 3,947), United States, 1996–2007

Characteristic	Age at First Birth						P for Interaction
	<20 Years			≥20 Years
	Preterm Birth		P Value	Preterm Birth		P Value
	No. of Participants	%a		No. of Participants	%a
Baseline age, yearsb	15.71 (0.22)		0.20	16.17 (0.21)		0.61	0.21
Baseline BMIb,c	22.85 (0.58)		0.29	22.81 (0.38)		0.05	0.93
Proximal BMIb,c	23.62 (0.73)		0.29	25.25 (0.50)		0.03	0.88
Baseline BMI category			0.47			0.19	1.00
Underweight	9	6.59		25	7.37
Normal weight	65	7.75		156	8.91
Overweight/obese	20	10.45		63	11.80
Proximal BMI category			0.39			0.07	0.60
Underweight	11	10.53		16	7.44
Normal weight	63	7.18		130	8.27
Overweight/obese	23	10.08		105	11.62
Race			0.63			0.54	0.95
Nonblack	68	7.99		187	9.13
Black	30	9.12		64	10.15
Ethnicity			0.23			0.03	0.05
Non-Hispanic	84	8.75		209	8.78
Hispanic	14	5.43		42	13.93
Gravidity			0.29			0.38	0.63
Primigravid	87	8.62		192	9.61
Multigravid	11	5.87		59	8.06
Household structure			0.54			0.30	0.97
2 biological parents in the home	40	9.18		131	10.10
Other	58	7.73		120	8.42
Parental educational level			0.68			0.98	0.76
High school graduate or higher	64	7.21		209	9.37
Less than high school graduate	24	8.18		34	9.42
Parental unemployment			0.92			0.97	0.96
Yes	75	8.54		212	9.60
No	10	8.98		11	9.73
Smoking during pregnancy			0.73			0.78	0.93
No	77	8.46		215	9.39
Yes	21	7.68		36	8.84
Received prenatal care			0.31			0.02	0.65
No	5	14.16		9	20.95
Yes	93	8.07		242	9.07
Initiation of prenatal care			0.96			0.02	0.22
First trimester	69	8.46		201	8.40
Second trimester	19	8.59		30	14.71
Third trimester	9	7.40		16	13.92
Conceived while using birth control			0.36			0.81	0.37
No	70	8.81		197	9.21
Yes	28	7.00		54	9.63
Marital status (married vs. unmarried)			0.14			0.46	0.13
Unmarried	79	7.42		132	9.85
Married	19	12.49		119	8.80
Marital status			0.03			0.29	0.21
Married	19	12.49		119	8.80
Cohabitated	32	11.29		77	11.15
Other	47	5.66		55	8.26

Abbreviations: BMI, body mass index; SE, standard error.

^a All percentages weighted for sampling design.

^b Mean (standard error).

^c Weight (kg)/height (m)².

Table 4.

Bivariate Analysis Between Maternal Characteristics and Birth Weight in the National Longitudinal Study of Adolescent Health (n = 3,947), United States, 1996–2007

Characteristic	Age at First Birth				P for Interaction
	<20 Years		≥20 Years
	Birth Weight, mean (SE)	P Value	Birth Weight, mean (SE)	P Value
Baseline age		0.35		0.15	0.10
Baseline BMIa		0.08		0.11	0.01
Proximal BMIa		0.12		0.41	0.07
Baseline BMI category		0.35		0.17	0.28
Underweight	3.16 (0.05)		3.27 (0.04)
Normal weight	3.26 (0.03)		3.29 (0.02)
Overweight	3.27 (0.07)		3.25 (0.05)
Obese	3.30 (0.10)		3.16 (0.06)
Proximal BMI category		0.05		0.03	0.40
Underweight	3.11 (0.06)		3.18 (0.04)
Normal weight	3.25 (0.03)		3.29 (0.02)
Overweight	3.35 (0.07)		3.30 (0.04)
Obese	3.22 (0.08)		3.19 (0.04)
Race		<0.01		<0.01	0.79
Nonblack	3.29 (0.03)		3.30 (0.02)
Black	3.09 (0.05)		3.08 (0.04)
Ethnicity		0.91		0.09	0.23
Non-Hispanic	3.24 (0.03)		3.28 (0.02)
Hispanic	3.25 (0.04)		3.20 (0.04)
Gravidity		0.01		0.57	0.02
Primigravid	3.22 (0.02)		3.27 (0.02)
Multigravid	3.39 (0.06)		3.28 (0.03)
Household structure		0.54		0.40	0.85
2 biological parents	3.27 (0.04)		3.28 (0.02)
Other	3.23 (0.03)		3.26 (0.02)
Parental educational level		0.57		0.11	0.13
High school graduate or higher	3.25 (0.03)		3.29 (0.02)
Less than high school graduate	3.28 (0.06)		3.20 (0.05)
Parental unemployment		0.43		0.13	0.10
No	3.25 (0.03)		3.27 (0.02)
Yes	3.30 (0.06)		3.19 (0.05)
Smoking during pregnancy		0.79		0.01	0.15
No	3.25 (0.03)		3.29 (0.02)
Yes	3.23 (0.05)		3.19 (0.04)
Received prenatal care		0.09		0.06	0.81
No	3.00 (0.15)		3.06 (0.11)
Yes	3.25 (0.02)		3.27 (0.02)
Initiation of prenatal care		0.89		0.35	0.55
First trimester	3.24 (0.03)		3.28 (0.02)
Second trimester	3.26 (0.05)		3.22 (0.06)
Third trimester	3.23 (0.08)		3.19 (0.07)
Conceived while using birth control		0.28		0.81	0.32
No	3.26 (0.03)		3.27 (0.02)
Yes	3.21 (0.04)		3.28 (0.03)
Marital status (married vs. unmarried)		0.57		<0.01	0.42
Unmarried	3.24 (0.02)		3.21 (0.02)
Married	3.28 (0.08)		3.32 (0.02)
Marital status		0.84		<0.01	0.71
Married	3.28 (0.08)		3.32 (0.02)
Cohabitated	3.23 (0.04)		3.20 (0.03)
Other	3.24 (0.03)		3.22 (0.04)

Abbreviations: BMI, body mass index; SE, standard error.

^a Weight (kg)/height (m)².

Table 5.

Bivariate Analysis Between Maternal Characteristics and Gestational Age in the National Longitudinal Study of Adolescent Health (n = 3,947), United States, 1996–2007

Characteristic	Age at First Birth				P for Interaction
	<20 Years		≥20 Years
	Gestational Age, weeks, mean (SE)	P Value	Gestational Age, weeks, mean (SE)	P Value
Baseline age, years		0.90		0.27	0.56
Baseline BMIa		0.93		0.12	0.44
Proximal BMIa		0.67		0.02	0.55
Baseline BMI category		0.99		0.40	0.90
Underweight	39.29 (0.19)		39.15 (0.13)
Normal weight	39.30 (0.11)		39.10 (0.07)
Overweight	39.23 (0.24)		38.86 (0.21)
Obese	39.19 (0.49)		38.76 (0.24)
Proximal BMI category		0.02		0.11	0.04
Underweight	38.77 (0.21)		39.24 (0.16)
Normal weight	39.35 (0.11)		39.13 (0.07)
Overweight	39.53 (0.21)		38.99 (0.16)
Obese	38.74 (0.40)		38.63 (0.21)
Race		0.27		0.24	0.91
Nonblack	39.33 (0.11)		39.08 (0.06)
Black	39.09 (0.18)		38.81 (0.22)
Ethnicity		0.50		0.06	0.10
Non-Hispanic	39.25 (0.11)		39.06 (0.06)
Hispanic	39.39 (0.16)		38.80 (0.13)
Gravidity		0.86		0.07	0.49
Primigravid	39.26 (0.09)		38.99 (0.07)
Multigravid	39.30 (0.22)		39.21 (0.10)
Household structure		0.22		0.06	0.98
2 biological parents in the home	39.11 (0.15)		38.92 (0.08)
Other	39.37 (0.12)		38.17 (0.10)
Parental educational level		0.94		0.31	0.55
High school graduate or higher	39.28 (0.11)		39.02 (0.07)
Less than high school graduate	39.30 (0.17)		39.19 (0.14)
Parental unemployment		0.74		0.94	0.78
No	39.27 (0.11)		39.04 (0.06)
Yes	39.36 (0.25)		39.01 (0.31)
Smoking during pregnancy		0.27		0.33	0.74
No	39.22 (0.10)		39.02 (0.07)
Yes	39.41 (0.16)		39.13 (0.10)
Received prenatal care		0.56		0.19	0.75
No	38.91 (0.63)		38.42 (0.47)
Yes	39.28 (0.09)		39.05 (0.06)
Initiation of prenatal care		0.91		0.29	0.34
First trimester	39.23 (0.11)		39.09 (0.06)
Second trimester	39.31 (0.19)		38.73 (0.26)
Third trimester	39.31 (0.28)		38.67 (0.37)
Conceived while on birth control		0.21		0.70	0.42
No	39.20 (0.11)		39.03 (0.07)
Yes	39.42 (0.15)		39.08 (0.11)
Marital status (married vs. unmarried)		0.10		0.89	0.11
Unmarried	39.37 (0.09)		39.03 (0.10)
Married	38.76 (0.35)		39.05 (0.07)
Marital status		0.05		0.76	0.28
Married	38.76 (0.35)		39.05 (0.07)
Cohabitated	39.10 (0.18)		38.97 (0.11)
Other	39.50 (0.10)		39.10 (0.19)

Abbreviations: BMI, body mass index; SE, standard error.

^a Weight (kg)/height (m)².

Younger teen mothers (age at birth <18 years) were also compared with a somewhat older adult group of mothers (age at birth ≥24 years); bivariate analyses were limited to continuous outcomes because fewer births occurred among this younger group of teen mothers (n = 335) (Web Table 1, available at http://aje.oxfordjournals.org/). Somewhat different patterns were seen with BMI in the 2 groups. For the younger women, the heaviest babies and longest gestations were in the obese group, whereas in the older women, the heaviest babies were on average in the underweight group (P for interaction = 0.02 for birth weight, 0.10 for gestational age).

In multivariable analysis (Appendix Table 1), among black adolescents, low parental educational levels and higher maternal age at pregnancy were associated with higher birth weight, whereas higher baseline age was associated with lower birth weight. Lower parental educational levels and being on birth control when one got pregnant were associated with higher gestational age (Appendix Table 2). In nonblack adolescents, lower BMI was associated with lower birth weight (Appendix Table 1), whereas being unmarried was associated with lower gestational age (Appendix Table 2).

Appendix Table 1.

Multivariable Analysis of Maternal Characteristics and Birth Weight in the National Longitudinal Study of Adolescent Health (n = 1,101), United States, 1996–2007

	Race
Characteristic	Black			Nonblack			P for Interaction
	Beta (kg)	SD	P Value	Beta (kg)	SD	P Value
Baseline age	−0.23	0.09	0.01	−0.00	0.06	0.94	0.02
Age at pregnancy	0.24	0.09	0.01	−0.02	0.06	0.74	0.01
Calendar year of pregnancy	−0.20	0.09	0.03	0.01	0.06	0.87	0.03
Baseline BMI category			0.63			0.01	0.19
Underweight	Referent			Referent
Normal weight	0.04	0.19		0.19	0.12
Overweight/obese	−0.09	0.16		0.21	0.07
Household structure			0.57			0.82	0.66
Other	0.09	0.16		0.02	0.07
2 biological parents	Referent			Referent
Parental educational level			0.00			0.51	0.01
Less than high school graduate	0.21	0.06		−0.05	0.08
High school graduate or higher	Referent			Referent
Gravidity			0.08			0.13	0.42
Multigravid	0.24	0.14		0.11	0.07
Primigravid	Referent			Referent
Smoking in pregnancy			0.35			0.47	0.24
Yes	0.13	0.14		−0.05	0.07
No	Referent			Referent
Received prenatal care			0.37			0.19	0.76
No	0.17	0.19		0.26	0.20
Yes	Referent			Referent
Conceived while using birth control			0.83			0.80	0.98
Yes	−0.02	0.08		−0.01	0.06
No	Referent			Referent
Marital status			0.20			0.83	0.20
Unmarried	−0.40	0.31		0.02	0.10
Married	Referent			Referent
Parental unemployment			0.98			0.97	0.97
Yes	0.00	0.13		0.00	0.08
No	Referent			Referent

Abbreviations: BMI, body mass index; SD, standard deviation.

Appendix Table 2.

Multivariable Analysis of Maternal Characteristics and Gestational Age in the National Longitudinal Study of Adolescent Health, United States (n = 1,101), 1996–2007

	Race
Characteristic	Black			Nonblack			P for Interaction
	Beta (weeks)	SD	P Value	Beta (weeks)	SD	P Value
Baseline age	0.25	0.35	0.47	0.00	0.24	0.99	0.56
Age at pregnancy	−0.17	0.38	0.65	−0.11	0.22	0.63	0.88
Calendar year of pregnancy	0.19	0.39	0.62	0.06	0.22	0.80	0.77
Baseline BMI category			0.44			0.43	0.22
Underweight	Referent			Referent
Normal weight	−0.41	0.64		0.41	0.34
Overweight/obese	−0.59	0.45		0.28	0.27
Household structure			0.13			0.86	0.18
Other	1.02	0.67		0.05	0.27
2 biological parents in the home	Referent			Referent
Parental educational level			0.03			0.39	0.02
Less than high school graduate	0.73	0.34		−0.21	0.24
High school graduate or higher	Referent			Referent
Gravidity			0.19			0.55	0.49
Multigravid	0.50	0.37		0.17	0.28
Primigravid	Referent			Referent
Smoking in pregnancy			0.94			0.69	0.80
Yes	−0.03	0.42		0.09	0.21
No	Referent			Referent
Received prenatal care			0.73			0.34	0.72
No	0.35	1.03		0.83	0.86
Yes	Referent			Referent
Conceived while using birth control			0.01			0.17	0.25
Yes	0.72	0.27		0.31	0.22
No	Referent			Referent
Marital status			0.40			0.04	0.72
Unmarried	−1.63	1.91		−0.93	0.45
Married	Referent			Referent
Parental unemployment			0.18			0.63	0.19
Yes	−0.57	0.42		0.18	0.38
No	Referent			Referent

Abbreviations: BMI, body mass index; SD, standard deviation.

DISCUSSION

Although we found that girls who became pregnant as adolescents were more likely to fit a high-risk profile than were girls who gave birth later, PTB and LBW were equally distributed between adolescent and young adult mothers. Whether adolescent mothers are truly at high risk for these complications has been a matter of some debate (2, 30–32). Adolescents with good access to health care often do not show a higher risk of complications (33, 34). The proportion of girls who reported becoming pregnant in this study (25%) is less than the national average. The national proportion of adolescents who became pregnant during the time period of this study (1994–2008) was between 30% and 40% (35). The study design was initially based in schools; this underrepresents girls who dropped out of school before wave I, a phenomenon that may be more likely among adolescents in the older age cohorts who become pregnant (29). It is also possible that there was selective attrition before wave IV because of childbearing, although weights constructed by Add Health researchers are meant to address differential loss to follow-up.

A few risk factors for lower birth weight and gestational age among adolescents were identified. Being married was associated with lower gestational age, consistent with research suggesting that adolescent parents may benefit from living with their family of origin (20). There was also a tendency for gestational age to be longer in girls who were using contraception when they conceived, especially among black women; higher risk with intended adolescent pregnancy has been previously reported (36). In bivariate analysis, rates of complications were higher in adolescents who did not get prenatal care, but this association was not as strong as in the older women.

Birth weight was lower in black adolescents than in white adolescents, but the disparity in LBW was similar to that seen in older women (14% vs. 8% in adults; 12.6% vs. 7% in adolescents), and absolute levels of LBW were lower in black teenagers than in older women, though not substantially (19, 22, 29). The interaction analysis suggested that the impact of the social context on birth outcomes might differ by ethnic group, as the relation with parental educational level and maternal age differed by racial group and associations were not necessarily in the expected directions. Greater birth weight by age at pregnancy within black adolescent mothers is consistent with past studies, which have found young black adolescent mothers to be at particular increased risk of adverse birth outcomes compared with their older peers (29). However, lower birth weight among black adolescent mothers with greater baseline age suggests possible cohort differences, such that earlier cohorts of these mothers were at increased risk of negative outcomes.

Strengths of the study include the large, representative sample and the prospective data collection, but some limitations need to be mentioned. The present study relies on self-reporting of birth outcomes (birth weight and gestational age). Although mothers' reports of these outcomes are generally reliable (37–40) and these pregnancies had occurred fairly recently, this is a potential source of error. Studies have found that maternal age does not affect the accuracy of reporting (39, 41), although ethnicity and socioeconomic status have been found to predict errors (42). Also, gestational age is ideally assessed with early ultrasound (43), especially for teenagers, who have more irregular menstrual cycles (44). For those who did not have such an ultrasound, the reported gestational age may have been inaccurate, even if the last menstrual period-based calculation is correctly reported. A large proportion of the study population was also using contraception when they conceived, which might have further delayed pregnancy recognition or contributed to errors in calculating last menstrual period. Given the multiple sources of error, the direction of bias is probably unpredictable. Also, the number of cases was relatively small, leading to imprecise estimates and limited power to examine interactions. We did not have any information on complications of pregnancy or whether a PTB was due to preterm labor, rupture of membranes, or medical indication.

The results of this analysis suggest that teenagers as a whole are not at increased risk of adverse birth outcomes compared with adult mothers despite their overall riskier behavioral and demographic profile. Further, the conventional risk factors, such as smoking and marital status, may not exert as strong effects on adolescent mothers as on adults. Access to prenatal care and support from family members appear to be potential intervention targets to improve birth outcomes among young mothers. Future research will address the social and environmental context of teen pregnancies and their relation with birth outcomes.