Population attributable risks for low birth
weight among singleton births—Colorado,
2007-2009
Ashley Juhl, MSPH
Epidemiology, Planning and Evaluation Branch
Prevention Services Division
Colorado Department of Public Health and Environment
CSTE
June 14, 2011
Outline
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Background on low birth weight
Purpose of this analysis
Methods for this analysis
Results of the analysis
Discussion of analysis results
Recommendations based on the results
Background and Purpose
Background
• 2009 low birth weight rate in Colorado = 8.8%
• Preliminary 2009 low birth weight rate for the
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United States = 8.2%
Low birth weight outcomes
– Neonatal mortality and morbidity
– Long-term developmental and neurological
disabilities and complications
Background
• Known risk factors for low birth weight
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Maternal smoking
Maternal alcohol consumption
Maternal age (<15 years and 35+ years)
Weight gain below the recommended range based on
prepregnancy BMI category (inadequate weight gain)
– Previous preterm birth
– Social and economic factors (low income, low educational
level, stress, unmarried, domestic violence or abuse)
– Infections and placental problems
Previous Colorado Study
1995-1997
• Population attributable risk analysis among
singleton births
• Inadequate weight gain during pregnancy (1 in 8 low
weight births)
• Smoking during pregnancy (1 in 8 low weight births)
• Inadequate weight gain and/or smoking (1 in 3 low
weight births)
http://www.cdphe.state.co.us/pp/womens/pdf/tippingthescales.pdf
Purpose
• To determine if the same risk factors still
contribute to low birth weight among
singletons
Methods
Methods
• Colorado birth certificates 2007-2009
– Cross-sectional study of a birth cohort
– Live births to Colorado residents
– Singletons only (N=202,498)
Variables
• Outcome of interest
– Low birth weight
• <2,500 grams (5 lbs, 8 oz or less)
Variables
• Risk factors
– Inadequate maternal weight gain
• Based on 1990 IOM recommendations
– Smoking during pregnancy
– Premature rupture of the membranes
– Pregnancy induced hypertension or eclampsia
– Abruptio placenta
– Short interpregnancy interval (<12 months between
previous delivery and subsequent conception)
– No prenatal care
– Placenta previa
– Altitude > 10,000 feet (maternal residence at birth)
Variables (continued)
• Combined risk factor
– Mothers gain inadequately
OR mothers smoke
OR mothers gain inadequately and smoke
Analysis Steps
Step 1. Calculated the prevalence of each risk factor
in the population
Step 2. Calculated low birth weight rate for each risk
factor
Step 3. Calculated relative risks (RR) and 95%
confidence intervals for each risk factor
Step 4. Calculated population attributable risk (PAR)
percent and 95% confidence intervals for each risk
factor
Results
Maternal Characteristics
Colorado 2007-2009 Singleton Births
Age (years)
<18
Mothers (%)
N=202,498
6,499 (3.2)
18-19
12,907 (6.4)
20-24
46,456 (22.9)
25-29
56,516 (27.9)
30-34
48,695 (24.0)
35+
31,408 (15.5)
Maternal Characteristics
Colorado 2007-2009 Singleton Births
Race/Ethnicity
Mothers (%)
N=202,498
White, non Hispanic
119,210 (58.9)
White, Hispanic
55,949 (27.6)
Black
9,680 (4.8)
Asian American/Pacific Islander
7,124 (3.5)
American Indian/Native Alaskan
2,061 (1.0)
Step 1. Prevalence
Colorado 2007-2009 Singleton Births
Risk Factor
Prevalence
Inadequate weight gain and/or smoking
26.2%
Inadequate weight gain during pregnancy
18.7%
Short interpregnancy interval
16.2%
Smoking during pregnancy
8.7%
Pregnancy induced hypertension or eclampsia
3.5%
Premature rupture of the membranes
2.5%
No prenatal care
1.5%
Abruptio placenta, placenta previa, and altitude >10,000 feet each had prevalence below 1%.
Step 2. Low Birth Weight (LBW) Rate
Colorado 2007-2009 Singleton Births
Risk Factor
LBW Rate
Premature rupture of the membranes
36.9%
No prenatal care
21.0%
Pregnancy induced hypertension or eclampsia
20.6%
Smoking during pregnancy
12.2%
Inadequate weight gain during pregnancy
10.5%
Inadequate weight gain and/or smoking
10.4%
Short interpregnancy interval
6.9%
Colorado singleton low birth weight rate = 7.0%
Step 3. Relative Risks (RR) for Low Birth Weight
Colorado 2007-2009 Singleton Births
Risk Factor
RR (95% CI)
Premature rupture of the membranes
5.91 (5.68-6.15)
No prenatal care
3.26 (3.03-3.50)
Pregnancy induced hypertension or
eclampsia
3.14 (2.99-3.30)
Inadequate weight gain and/or smoking
1.91 (1.85-1.98)
Smoking during pregnancy
1.87 (1.79-1.95)
Inadequate weight gain during pregnancy
1.79 (1.73-1.86)
Short interpregnancy interval
1.15 (1.08-1.22)
Step 4. Population Attributable Risks (PAR) for
Low Birth Weight
Colorado 2007-2009 Singleton Births
Risk Factor
PAR (95% CI)
Inadequate weight gain and/or smoking
19.4% (18.2-20.5)
Inadequate weight gain during pregnancy
12.9% (12.0-13.9)
Premature rupture of the membranes
11.1% (10.5-11.6)
Smoking during pregnancy
7.1% (6.4-7.7)
Pregnancy induced hypertension or
eclampsia
6.9% (6.4-7.4)
No prenatal care
4.2% (3.7-4.6)
Short interpregnancy interval
2.4% (1.3-3.4)
Discussion and Recommendations
Discussion
• Inadequate maternal weight gain: 1 in every 8 low
weight births
• Smoking during pregnancy: 1 in every 14 low weight
births
• Compared to the 1995-1997 study, no change in
inadequate weight gain contribution, but a large drop
in smoking contribution
• Smoking prevalence declined 25%
Conclusions
• Inadequate weight gain and smoking during
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pregnancy still contribute to low birth weight
Prevalence of smoking during pregnancy declined
which decreased its contribution to low birth weight
Public health implication
– If programmatic efforts move pregnant women
towards adequate weight gain and increase
smoking cessation prior to conception, low birth
weight births could be reduced.
– Hypothetical state low birth weight rate = 7.5%
• Actual 2009 LWB rate = 8.8%
Strengths & Limitations
• Strengths
– Large birth cohort to study
• Able to stratify
• Limitations
– Not all LBW risk factors are included on birth
certificate
• Example: maternal stress
Recommendations for Colorado
• Continue to
– Counsel women on smoking cessation before
conception and during pregnancy
– Encourage women to gain an adequate amount of
weight during pregnancy
Recommendations for All
Future Tasks
• Focus public health efforts to improve
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preconception and interconception health
Opportunities for program collaboration
Acknowledgments
• Colorado Department of Public Health and Environment
– Barbara Gabella. MSPH
– Indira Gujral, MS, PhD
– Sue Ricketts, MA, PhD
– Linda Archer, MSN, RN, CNS
– Mandy Bakulski, RD
– Kirk Bol, MSPH
– Epidemiology, Planning and Evaluation Branch
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The Prenatal Program of the Women's Health Section and Grant Number B04MC17035 from
the Maternal and Child Health Bureau (Title V, Social Security Act), Health Resources and
Services Administration, Department of Health and Human Services supports this project.
The findings and conclusions in this presentation
are those of the author(s) and do not necessarily
represent the views of the Colorado Department
of Public Health and Environment.
Thank You
Questions & Comments
ashley.juhl@state.co.us
References
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Births: Preliminary data for 2009. National Center for Health Statistics.
Ricketts S, Trierweiler K, Colorado. Dept. of Public Health and Environment.
Tipping the scales : weighing in on solutions to the low birth weight problem in
Colorado. Denver, Colo. (4300 Cherry Creek Drive South, Denver 80246):
Colorado Dept. of Public Health & Environment; 2000.
Colorado Birth Certificates, Vital Statistics Section, Colorado Department of
Public Health and Environment.
Centers for Disease Control. National Environmental Public Health Tracking
Network. http://ephtracking.cdc.gov/showIndicatorsData.action.