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Maternal Smoking and Infant Birth Weight

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Maternal Smoking and
Infant Birth Weight
Lucinda England, MD, MSPH
National Institute of Child
Health and Human
Development (NICHD)
Lucinda England, MD, MSPH
–Post-doctoral Fellow in the Division
of Epidemiology, Statistics, and
Prevention Research, NICHD
–Interests: smoking and fetal growth,
smoking and preeclampsia
–This research was conducted while
the author was an Epidemic
Intelligence Officer at the Centers
for Disease Control and Prevention
Learning Objectives
пЃ® Understand
the relationship
between tobacco exposure
during pregnancy and fetal
growth
пЃ® Understand the limitations of
methods currently available for
measuring tobacco exposure
Learning Objectives
(Continued)
пЃ® Appreciate
the importance of
understanding the nature of the
relationship between exposure
and outcome
Smoking during pregnancy
increases likelihood of
пЃ®
Low birth weight
пЃ®
Intrauterine growth restriction
Preterm delivery
SIDS
Maternal complications
пЃ®
пЃ®
пЃ®
Quitting during pregnancy
improves birth weight, but...
пЃ®
пЃ®
пЃ®
Only one-in-four pregnant
smokers quits successfully
One-in-three pregnant
smokers cuts back
It is unknown if cutting back is
beneficial
How do we measure tobacco
exposure?
1. Self report: Inexpensive, but
has several limitations
a) Some may not report use
honestly
b) Recall bias
c) People tend to round to the
nearest pack or half-pack
How do we measure tobacco
exposure?
2. Biomarkers: Objective way to
validate and quantify exposure
a) Thiocyanate
b) Carbon monoxide
c) Cotinine
Biomarkers: cotinine
пЃ®
Primary metabolite of nicotine
пЃ®
Popular biomarker because it:
-Has long half life (20 hours)
-Can be detected in serum,
urine, or saliva
-Is stable in stored specimens
Unresolved research issues:
1. Can cotinine concentration
be used to predict how much
a woman smokes?
2. Does a pregnant woman
who cuts back improve her
infant’s birth weight?
To address these issues, we
need to know the following:
1. What is the nature of the
relationship between cotinine
and cigarettes per day?
2. What is the nature of the
relationship between tobacco
exposure and birth weight?
Methods
Data source: A collaborative
project between the CDC and
three state health departments
Study population: Self-reported
smokers attending public
prenatal clinics from 1987-1990
and giving birth to live, term,
singleton infants
Outcome = birth weight
Exposure = tobacco
– Measured in two ways:
• Self-reported cigarettes
smoked per day
• Urine cotinine concentration
– Obtained at two points in time:
• At entry into prenatal care
• In the 3rd trimester
Analysis I: Relationship
between cigarette smoking
and urine cotinine
пЃ® General
linear models were
used to quantify the amount of
cotinine variability explained by
self reported cigarettes smoked
per day.
Analysis II: Tobacco exposure
and birth weight
пЃ® General
linear models were
used to generate mean adjusted
birth weight.
пЃ® Birth weight was plotted against
cigarettes per day and against
urine cotinine concentration.
Results
Study population
пЃ® 5,572
women screened
пЃ® 4,005 women eligible
пЃ® 3,395 women had at least one
set of corresponding measures
of exposure
пЃ® 2,481 women still smoking in
3rd trimester
Cigarettes per day and urine
cotinine (See Figure 1)
пЃ® Median
urine cotinine
concentration increased as
cigarettes per day increased.
пЃ® Urine cotinine concentration
reached a plateau at > 15
cigarettes per day.
Cigarettes per day and urine
cotinine (See Figure 1)
пЃ® Urine
cotinine concentration
varied considerably at all levels
of cigarette smoking.
пЃ® The amount of variation in urine
cotinine concentration
explained by cigarettes per day
(r2) was only 14 percent.
Figure 1. Box and whiskers plot of
urine cotinine by cigarettes per day
Click for larger picture
Tobacco exposure and birth weight
пЃ® Mean
adjusted birth weight
decreased as cigarettes per day
increased (See Figure 2).
пЃ® The relationship was not linear: the
sharpest decline in birth weight
seen at low levels of smoking.
пЃ® A similar pattern seen with cotinine
concentration (see Figure 3).
Figure 2. Mean adjusted infant birth
weight by 3rd trimester smoking
Click for larger picture
Figure 3. Mean adjusted infant birth
weight by 3rd trimester cotinine
Click for larger picture
Discussion
Answers to research
questions:
1. What is the relationship
between urine cotinine and
cigarettes smoked per day?
Answer: Urine cotinine
concentration does not correlate
closely with cigarettes per day.
Answers to research
questions:
2. What is the relationship
between tobacco exposure
and birth weight?
Answer: Birth weight declines
sharply at low levels of
exposure, then levels off.
Study Limitations
пЃ® We
don’t know time elapsed
between last cigarette smoked
and urine collection.
 We don’t have information on
types of cigarette smoked or
how cigarettes were smoked.
Conclusions
пЃ® Our
ability to quantify tobacco
exposure is limited; both selfreported cigarette use and urine
cotinine have serious limitations.
пЃ® More research is needed to
uncover the mechanisms
through which tobacco causes
fetal growth restriction.
Conclusions
пЃ® Studies
of the effects of
smoking reduction on health
outcomes are needed before
recommending reduction as
an alternative to quitting.
Suggested readings
England et al. Measures of maternal tobacco exposure and
infant birth weight at term. Am J Epidemiol, 2001;153:954-60.
England et al. Effects of smoking reduction during pregnancy
on the birth weight of term infants. Am J Epidemiol, (in press).
Ellard GA et al. Smoking during pregnancy: the dose
dependence of birthweight deficits. Br J Obstet Gynaecol,
1996; 103:806-13.
Klebanoff MA et al. Serum cotinine concentration and selfreported smoking during pregnancy. Am J Epidemiol, 1998;
148:259-62.
Haddow JE et al. Cigarette consumption and serum cotinine
in relation to birth weight. Br J Obstet Gynaecol, 1987;94:678-81.
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