Objective
The aim of this study was to determine whether inadequate prenatal care is associated with increased risk of preterm birth among adolescents.
Study Design
We selected a random sample of women under age 20 years with singleton pregnancies delivering in Washington State between 1995 and 2006. Multivariate logistic regression was used to assess the association between prenatal care adequacy (percent of expected visits attended, adjusted for gestational age) and preterm birth.
Results
Of 30,000 subjects, 27,107 (90%) had complete data. Women without prenatal care had more than 7-fold higher risk of preterm birth (n = 84 [24.1%]; adjusted odds ratio [aOR], 7.4), compared with those attending 75-100% of recommended visits (n = 346 [3.9%]). Women with less than 25%, 25-49%, or 50-74% of expected prenatal visits were at significantly increased risk of preterm birth; risk decreased linearly as prenatal care increased (n = 60 [9.5%], 132 (5.9%], 288 [5%]; and aOR, 2.5, 1.5, and 1.3, respectively).
Conclusion
Inadequate prenatal care is strongly associated with preterm birth among adolescents.
The US teenage pregnancy rate is one of the highest among industrialized nations. Although the rate of teen pregnancy declined between its peak of 61.8 births per 1000 teens aged 15-19 years in 1991 and reached a low of 40.5 in 2005, preliminary data for 2006-2007 show that over those 2 years, rates have risen to 42.5 births per 1000 girls aged 15-19 years.
For Editors’ Commentary, see Table of Contents
Data suggest that pregnant teenagers are more likely than adult women to suffer adverse medical and obstetric outcomes, such as hypertensive disease, anemia, infection, and depression, during pregnancy and may continue to have consequences, like depression later in life, delayed or discontinued education, or increased utilization of public assistance. Age younger than 17 years is associated with a 1.5-1.9 times increased risk of preterm birth. Preterm birth, defined as delivery before 37 weeks’ gestational age, affects more than 10% of live births annually in the United States and is responsible for three-quarters of all neonatal mortality and 35% of all health care spending for infants in the United States.
Prenatal care may decrease adverse pregnancy outcomes for teenage pregnant women by reducing risk factors through education and social support. Teenage and adult mothers probably differ in their access to and utilization of prenatal care. We hypothesize that inadequate prenatal care will increase the risk of preterm birth for adolescents.
Materials and Methods
We used Washington State birth record data to conduct a population-based cohort study of women who delivered between the years 1995 and 2006. Eligible subjects were women under 20 years of age who had singleton births during the study period. From this population, 30,000 women were selected at random for inclusion. Women with pregnancies affected by fetal malformations or chromosomal abnormalities were excluded, as were women with recorded gestational age at delivery greater than 43 weeks. The primary outcome was delivery at less than 37 weeks. The primary exposure of interest was adequacy of prenatal visits. The study received approval from the University of Washington Institutional Review Board.
Preterm birth was defined as gestational age less than 37 weeks at the time of delivery. To classify adequacy of prenatal care, we calculated a ratio of the actual number of prenatal visits compared with the expected number of visits for a delivery at a given gestational age. We used the American College of Obstetrics and Gynecology guidelines for the schedule of prenatal care visits to calculate the expected number of visits: every 4 weeks from the first prenatal visit through 28 weeks, every 2-3 weeks from 28 weeks until 36 weeks, and weekly thereafter.
We created a ratio of observed to expected visits, similar to Kotelchuck’s prenatal care index (APNCU-Adequacy of Prenatal Care Utilization Index). For the purposes of analysis, we divided adequacy of prenatal care into 6 categories: no prenatal care, less than 25%, 25-49%, 50-74%, 75-100%, and greater than 100% of expected. We used an observed to expected visit ratio of 75-100% of prenatal visits as our referent or ideal category. Other data included maternal age, race, insurance, smoking, first-trimester bleeding, prior preterm birth, pregestational diabetes, hypertensive disease, and culture-positive Neisseria gonorrhea or Chlamydia trachomatis . Data for N gonorrhea and C trachomatis were available only after 2002.
All analyses were conducted using Stata 10.0 (Stata Corp, College Station, TX). One-way ANOVA and χ 2 tests were used to test for differences in demographic, reproductive, and behavioral variables across the 6 exposure categories of prenatal care adequacy. Univariate logistic regression was used to assess the crude association between prenatal care and preterm birth.
In the multivariate analysis, variables that were potential confounders based on the univariate analysis or were strongly associated with preterm birth in the literature were included in all models (maternal age, race/ethnicity, marital status, maternal smoking, and prior preterm birth); subjects missing data for any of these variables were excluded from all analyses.
A subgroup analysis by teen age groups (maternal age ≤15 years, 16-17 years, and 18-19 years) was conducted, as was a multinomial logistic model with 3 categories of birth outcome (<32 weeks, 32-36 weeks, 37+ weeks). We conducted a stratified analysis in 3 year blocks to assess change in risk during the study period.
To detect a 30% difference in the rate of preterm birth for teens with poor prenatal care (80% power and significance level of alpha = 0.05), we required a sample size of 15,000. There are approximately 8000 live births to teens annually in Washington State. Assuming that 20% of all births in Washington State would have the variables of interest recorded completely, it was presumed that there would be data on 1600 teen births annually. Therefore, we determined that we needed to review approximately 10 years of data to detect a 30% difference in the rate of preterm birth for teens with poor prenatal care.
Results
Of the random selection of 30,000 women under age 20 years with singleton births in Washington State from 1995 to 2006, 27,107 (90%) had complete data and were included in this analysis. We excluded 642 subjects (2%) because their pregnancies were affected by fetal malformations (n = 634) or their recorded gestational age at delivery was greater than 43 weeks (n = 8). An additional 2251 subjects (8%) were excluded for missing maternal race, marital status, maternal smoking, and prior preterm birth or parity variables.
The overall rate of preterm birth in this population was 7% ( Table 1 ). A total of 349 women received no prenatal care, whereas 8983 attended 75-100% of expected visits. For teens with no prenatal care, 24.1% of births were preterm, compared with 3.9% preterm births with 75-100% of visits and 10.5% preterm births for more than 100% of visits. Eighty-eight percent of preterm births occurred between 32 and 36 weeks’ gestational age.
| Variable | Percentage of observed/expected prenatal visits | ||||||
|---|---|---|---|---|---|---|---|
| None, n (%) | <24%, n (%) | 25-49%, n (%) | 50-74%, n (%) | 75-100%, n (%) | >100%, n (%) | Total, n (%) | |
| Total births | 349 (1.3) | 629 (2.3) | 2254 (8.3) | 5718 (21.1) | 8983 (33.1) | 9174 (33.8) | 27,107 |
| >37 weeks | 265 (75.9) | 569 (90.5) | 2122 (94.1) | 5430 (95.0) | 8637 (96.1) | 8214 (89.5) | 25,237 (93.1) |
| <37 weeks | 84 (24.1) | 60 (9.5) | 132 (5.9) | 288 (5.0) | 346 (3.9) | 960 (10.5) | 1870 (6.9) |
| 32-36 weeks | 62 (17.8) | 54 (8.6) | 119 (5.3) | 252 (4.4) | 320 (3.6) | 841 (9.2) | 1648 (6.1) |
| <32 weeks | 22 (6.3) | 6 (0.9) | 13 (0.6) | 36 (0.6) | 26 (0.3) | 119 (1.3) | 222 (0.8) |
Women with inadequate prenatal care were younger and more likely to be unmarried, nulliparous, have government or charity-funded insurance, smoke during pregnancy, and have a history of preterm birth ( Table 2 ). Women with first-trimester bleeding, pregestational diabetes, and preeclampsia were more likely to have attended more than 100% of anticipated prenatal care visits. Rates of chronic hypertension were similar between groups. There was no significant difference in N gonorrhea or C trachomatis prevalence by prenatal care visit category for years with available data.
| Characteristic | Percentage of observed/expected prenatal visits | ||||||
|---|---|---|---|---|---|---|---|
| None (n = 349) | <25% (n = 629) | 25-49% (n = 2254) | 50-74% (n = 5718) | 75-100% (n = 8983) | >100% (n = 9174) | P value b | |
| Age, y | 17.5 ± 1.4 | 17.6 ± 1.4 | 17.7 ± 1.4 | 17.8 ± 1.3 | 17.9 ± 1.2 | 17.9 ± 1.2 | < .001 |
| Single | 313 (90) | 514 (82) | 1784 (79) | 4372 (77) | 6857 (76) | 6870 (75) | < .001 |
| Nulliparous | 272 (94) | 427 (92) | 1710 (95) | 4580 (97) | 7599 (98) | 7811 (98) | < .001 |
| Race | |||||||
| White | 202 (58) | 293 (47) | 1219 (54) | 3458 (61) | 6125 (68) | 6558 (72) | < .001 |
| Black | 31 (9) | 53 (8) | 170 (8) | 390 (7) | 449 (5) | 465 (5) | |
| Other | 116 (33) | 283 (45) | 865 (38) | 1870 (33) | 2409 (27) | 2151 (23) | |
| Uninsured c | 121 (62) | 331 (80) | 1086 (76) | 2669 (71) | 4132 (67) | 4092 (63) | < .001 |
| Smoking | 94 (27) | 147 (23) | 486 (22) | 1223 (21) | 2023 (23) | 2150 (23) | .017 |
| Chronic hypertension | 1 (<1) | 0 | 10 (<1) | 18 (<1) | 26 (<1) | 43 (<1) | .197 |
| Preeclampsia | 18 (5) | 15 (2) | 112 (5) | 305 (5) | 546 (6) | 742 (8) | < .001 |
| First-trimester bleeding | 5 (1) | 3 (0.5) | 19 (0.8) | 51 (0.9) | 82 (0.9) | 128 (1.4) | .005 |
| Diabetes | 2 (<1) | 5 (1) | 27 (1) | 57 (1) | 100 (1) | 165 (2) | < .001 |
| C trachomatis d | 6 (4) | 10 (5) | 41 (5) | 79 (4) | 114 (4) | 113 (4) | .924 |
| N gonorrhea d | 1 (<1) | 0 | 4 (<1) | 4 (<1) | 4 (<1) | 8 (<1) | .386 |
| Prior preterm birth e | 3 (1) | 3 (0.5) | 8 (0.4) | 31 (0.5) | 32 (0.4) | 46 (0.5) | < .001 |
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