Does Caffeine Consumption During Pregnancy Increase the Risk of Miscarriages or Low
Birth Weight in Humans?
[authors]
An argumentative paper for women of reproductive age
A paper for Pregnancy & Newborn magazine
The Connection Between Caffeine and Pregnancy: Why You Should Care
Women put forth a large amount of energy, time, and money to ensure that their
babies are happy and healthy. From numerous doctor visits to restrictive diets, women
strive to be the best mothers they can be from the moment of conception. If you are
pregnant, may become pregnant, or have a loved one who is pregnant, it is important that
you are properly educated on how prenatal consumption can affect an unborn child.
Typically, pregnant women are advised to abstain from alcohol, nicotine, and
caffeine. While nicotine and alcohol are proven to have harmful effects, caffeine’s
effects are less certain.
Caffeine could have possible negative effects on an unborn child such as
miscarriage, low birth weight, birth defects, early gestational age, and behavioral issues
in a child. While we understand that these outcomes can be devastating on the mother and
child, this paper will focus on just miscarriage and birth weight. Among the possible
effects, miscarriages and birth weights are common for researchers to evaluate
throughout the pregnancy and after delivery. Also, by looking at these two possible
outcomes, we will be evaluating the effects of caffeine from an extreme risk
(miscarriages) to a less life threatening risk (low birth weight). Because these two
outcomes are on opposite sides of the spectrum, it allows us to encompass other effects.
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For example, infants born prematurely or with a birth defect usually are born underweight
as well. By examining studies that focused on miscarriage or birth weight we will be able
to better understand the effects of caffeine on an unborn child.
Through looking at these two outcomes, we will be determining the best
conclusion to the question: does caffeine consumption during pregnancy increase the risk
of miscarriages or low birth weight? In determining the best conclusion, we analyzed
conflicting data from nine studies and one review that included seven studies (Figure 1).
These studies examined women who ranged in age, marital status, and health. The
researchers monitored the women’s caffeine consumption using a phone interview or
questionnaire throughout their pregnancy or after their delivery or miscarriage. We found
four studies that looked at the effects of caffeine on birth weight, four studies that looked
at the effects of caffeine on miscarriages, and one study, Mills et al., that looked at both
miscarriages and low birth weight.
In order to understand how researchers arrived at their conclusions it is important
to know how they examined caffeine. First, caffeine comes in many forms. The major
sources of caffeine are coffee, tea, chocolate/cocoa, medication and soft drinks, the
biggest source for most people being coffee. Studies varied in how they quantified
caffeine consumption; some used cups and others used converted cups to milligrams. In
order to standardize the results, we chose to separate caffeine consumption into
categories of low, medium, and high based on milligrams. For our evaluation, we
considered a low dose of caffeine to be anywhere between 0 to 150 mg a day. A medium
dose of caffeine would be between 150 to 300 mg a day, and a high dose would be
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anything above 300 mg of caffeine a day. In most cases, one cup of coffee contains about
150 mg of caffeine, a cup of tea contains 40 mg, and a soft drink contains 50 mg (Heller).
Another way to understand how researchers reached their conclusion is based on
how they interpreted their results. The health of an infant and the success of pregnancies
can be influenced by many factors, making it difficult for researchers to focus on the
effects of any one substance, such as caffeine. In order to account for the other factors
many studies used an adjusted odds ratio (OR). This ratio is calculated from the data in
order to “remove” factors that the scientists are not trying to study such as the mother’s
alcohol consumption, nicotine intake, or health predispositions. In these studies, the OR
indicates the likelihood that miscarriages or low birth weight occurred due to caffeine
alone. The baseline value for an OR is 1, which indicates women who consumed no
caffeine. As the OR increases, it means that the likelihood of having a miscarriage or
underweight baby due to just caffeine consumption also increases. This system can be
seen by the graphs on the left in Figure 2 and 3. Another way to interpret the data is to
simply take the average birth weights of infants or the percent of miscarriages and see if
the numbers are higher or lower due to caffeine consumption. Examples of these different
measures can be seen by the graphs on the right in Figures 2 and 3.
What Researchers are Saying and What We Think
The data regarding the effects of caffeine on miscarriages and birth weight are
conflicting and unresolved. Figure 1 shows five studies that examined caffeine’s effect on
birth weight and five studies that examined caffeine’s effect on miscarriages. Some of
these studies found that caffeine affects miscarriages or birth weight, while other studies
found that caffeine does not have an effect. In the Fenester et al. study, women who
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consumed a high dose of caffeine (300+ mg) had an OR of 2.36, which means they were
2.36 times more likely to have an underweight infant (Figure 2, red). Similarly, in the
Cnattingius et al. study, researchers found that women who consumed a high dose of
caffeine were 2 times more likely to have a miscarriage (Figure 2). When comparing
these ORs to the base value of 1, they are significantly higher and therefore these
researchers concluded that caffeine has an effect on miscarriages or birth weight. In
contrast, Mills et al. calculated that women who consumed a high dose of caffeine were
only 1.15 times more likely to have an underweight infant (Figure 3, purple). Similarly,
the Clausson et al. study found that infants had an average birth weight of 8.069 lbs,
8.078 lbs, and 8.00 lbs in women who consumed a low, medium, and high dose of
caffeine respectively (Figure 2, green). These averages were almost identical regardless
of caffeine consumption. In both these studies, the researchers found that caffeine does
not have an effect on miscarriages or birth weight. Although, all of these studies
monitored caffeine intake and miscarriages or birth weight, they reached opposite
conclusions proving the data is conflicting and thus unresolved.
Although the data is conflicting, we will analyze the studies thoroughly in order to
find if the evidence is stronger for one conclusion than for the other. At this stage of
research, the evidence is pointing towards the conclusion that consuming caffeine while
pregnant does not increase the risk of miscarriages or low birth weights. This is due to the
fact that these studies were typically designed in a more reliable way that reduced the
chance for possible errors to occur. Due to the conflicting data, there is an alternative
conclusion that caffeine has an effect on miscarriages or birth weight. This is
strengthened by the fact that more studies reached this conclusion. However, one very
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reliable study is far more beneficial in reaching a conclusion than multiple unreliable
studies and you will see that throughout this paper.
Comparing: The Start of Determining Reliability
In determining the reliability of the studies, we compared them in groups based on
their conclusions in order to see the strengths and limitations. One group would include
the studies that found that caffeine has an effect and the other group would include the
studies that concluded caffeine does not have an effect on miscarriages or birth weight.
First we did a comparison between the different types of caffeine researchers accounted
for. As we discussed above, caffeine can come in many different forms. Some studies,
such as the Cnattingius et al., focused on nine different types of caffeine while other
studies, such as Bech et al. and Armstrong et al., just looked at caffeine in the form of
coffee. In Figure 4, you can see that the types of caffeine the researchers accounted for
varied widely between all nine studies but not necessarily between each conclusion. The
studies that concluded caffeine does not have an effect on miscarriages or birth weight
accounted for an average of 4.5 types of caffeine, and the studies that found that caffeine
has either effect accounted for an average of 4.2. Since these averages are almost
identical, it does not strengthen one conclusion over the other.
Another comparison that could impact the reliability of these studies is how they
estimated caffeine consumption. In these studies, researchers asked mothers how much
caffeine they ate or drank per day. Then for the beverages, the researchers would either
keep their answers in terms of cups or convert them into milligrams (mg). A standard cup
of coffee can range from 29-333 mg of caffeine due to differences in caffeination or cup
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size (Heller). This large range can complicate the accuracy of using milligrams or cups as
a form of measurement and lead to unreliable results.
These studies used different methods for estimating caffeine in coffee and other
forms. For coffee some studies, such as Clausson et al. and Cnattingius et al., estimated
the caffeine based on how it was made: brewed, boiled or instant. They estimated that
there is 115 mg if the coffee was brewed, 90 mg if boiled, and 60 mg if instant. However,
most studies just used one estimation regardless of how it was made for coffee and other
forms of caffeine and these varied. As you can see by Figure 5, there is a large difference
in caffeine estimation between studies. Fenester et al. (Figure 5, green) estimated that
there is 47 mg of caffeine in a soft drink while Vik et al. (Figure 5, orange) estimated
there is only 13 mg of caffeine per soft drink. The variability in these estimations creates
a limitation in all nine studies, which makes it hard to reach a definitive solution. Our
hope was to find a trend in the way researchers estimated caffeine and the conclusion that
they reached, but there was none. Figure 5 shows that regardless of whether the
researchers had high or low estimation of caffeine, it did not dictate if they found that
caffeine has an effect or not. Therefore, the ways in which researchers estimated for
caffeine did not impact our conclusion.
Furthermore, this comparison allowed us to determine if simply using cups rather
than converting to milligrams affected the outcome of a study. There were three studies
that did not convert to milligrams and kept their data in terms of cups: Bech et al.,
Howard et al., and Armstrong et al. Two of these studies concluded that caffeine does not
have an effect and one study concluded that caffeine has an effect on miscarriages or
birth weights. Based on this, varying methods involving the conversion of cups to
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milligrams or just using cups did not impact either their conclusion or ours. Although
this did not help us reach our conclusion, it was important to compare these studies in
multiple ways to see if there were comparisons that strengthened one conclusion over the
other.
The Other Side
One comparison that weakens our argument is that more studies concluded that
caffeine consumption has an effect than concluded that it does not have an effect on
miscarriages or birth weight. Out of the five studies that looked at the effects of caffeine
on birth weight and the five studies that were included in the review, six studies found
that caffeine increased the risk of having an underweight child (Figure 1). Similarly, out
of the five studies that looked at the effects of caffeine on miscarriages and the two
studies that were included in the review, four found that there was an effect. This means
that out of 17 studies total, 10 found that caffeine has an effect on either miscarriages or
low birth weight. Typically we are taught that majority rules, and this caused us to
originally believe that caffeine has an effect. However, through closer comparisons of
these studies we found far more aspects that weakened the conclusion that caffeine has an
effect than that strengthened it, which will be shown in the rest of this paper.
Would You be Able to Remember a Cup of Coffee Nine Months Ago?
A difficulty with all these studies is that they rely on the ability of women to
recall their caffeine consumption throughout their pregnancy. The accuracy and
credibility of this recall relies heavily on whether the study had a retrospective or a
prospective design. A retrospective design is when researchers interview women about
their caffeine consumption after their delivery or miscarriage. A prospective design is
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when researchers interview women at their first prenatal appointment or before
conception and then continue to interview them throughout their pregnancy. As you can
imagine, it would be more difficult to recall your consumption of coffee, tea, soft drinks,
or chocolate over the past nine months than it would be to recall your consumption from
last week. Thus, prospective studies are stronger than retrospective studies.
We found that studies that concluded caffeine has either effect typically used the
less reliable retrospective design while the studies that concluded caffeine does not have
an effect typically used the more reliable prospective design (Figure 6). Out of the five
studies that found caffeine has either affect, the majority were retrospective (four out of
five). For example, the Fenester et al. study, did not interview mothers about their
caffeine consumption until nine months after delivery. This means that they were asking
women to recall all the caffeinated beverages they had drank for the past 18 months! If
every woman were to forget drinking just one cup of coffee (150 mg) a week, the
researchers’ data would be off by about 5400 mg of caffeine per pregnancy. It is clear
that this could lead to inaccuracies. Since the majority of these studies used this design it
lessens the credibility of their conclusion that caffeine has an effect on miscarriages or
birth weight.
In contrast, only one of the five studies that found caffeine does not have an effect
used a retrospective design. In this case, the majority of studies that concluded caffeine
does not have an effect used a prospective design (Figure 6). In the Mills et al. study, they
had women fill out a questionnaire about their caffeine consumption at their first prenatal
appointment. These questionnaires were then repeated at weeks 6, 8, 10, 12, 20, 28, and
36. This was an extremely thorough assessment and the researchers came to the overall
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conclusion that “despite [their] intensive surveillance” caffeine does not have an effect on
an unborn child. The majority of these studies used similar approaches and reached the
same conclusion. Asking women eight times during a nine month period versus one time
after 18 months has such a huge influence on the credibility of a study that it swayed us
to conclude that caffeine does not have an effect on miscarriages or low birth weight.
The Bigger the Study the Better
The number of people in a study is important because the purpose of a study is to
reach a conclusion that applies to the whole population. Since researchers cannot feasibly
include the entire population in their study, they rely on a group to represent it. The larger
this group is the more it will represent the population. Also, by having a larger group,
unusually high or low results will cancel each other out and create an average. If a study
has too few subjects, this average would be affected by any unusually high or low result.
In the end you want to know if the study can be applied to you. If studies have more
people in them, they will do a better job of accounting for the population and variability
in data, which increases the likelihood that the results found can be applied to you.
To see which studies were more representative of the population, we compared
group size between the studies. In addition to having a better design, studies that
concluded caffeine does not have an effect had more people than those that concluded
caffeine has an effect on miscarriages or birth weight. In the studies that found caffeine
does not have an effect there were 134,182 women but in the studies that found either
effect there were only 89,997 women (Figure 7). Not only did the studies that found
caffeine has an effect have fewer women, 98.3% of these women were from the Bech et
al. study. Although this strengthens the reliability of this one study, the remaining four
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studies are very small which weakens the overall conclusion more. Such a small size in
these others studies causes the conclusion to be easily affected by unusual results and
limits its ability to be applied to the population. In contrast, two large studies concluded
that caffeine does not have an effect as opposed to only one study concluding that it does
(Figure 8). The Howard et al. study had 97,903 women, and the Armstrong et al. study
had 35,848 women. The large number of people in these studies makes the results more
applicable to the population. Due to the fact there were two large studies and overall
more people in the studies that concluded caffeine does not have effect, we are even more
confident in our conclusion.
The Big Picture: What Should I Do?
Through our evaluation of these studies it is evident that there is not a definitive
conclusion. This is an issue that has many different components, which makes it difficult
for the studies to be identical and thus harder to compare the studies to understand how
the researchers are getting contrasting conclusion. As much as we want to be able to
make a definitive conclusion about caffeine’s effect on an unborn child, we cannot. In
order to make this claim, we need more studies that have a large amount of people, use a
prospective design, and estimate for caffeine in a uniform way. Until we have more
studies like this, the best we can do is evaluate the current studies to see where the
strengthens and weaknesses lie. Through our evaluation, we found that the studies that
concluded caffeine does not have an effect on miscarriages or birth weight were typically
more reliable studies and thus, their conclusions were stronger. In contrast, the studies
that found caffeine has an effect were designed in a way that allowed for more errors and
inaccuracies. These inaccuracies could have impacted their results, thus weakening their
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overall conclusion. Because of this we concluded that caffeine does not have an effect on
miscarriages or birth weight. However, the decision to consume or abstain from caffeine
while pregnant is up to you. If you want to play it safe, we suggest you do not consume
any type of caffeine while pregnant. Regardless of what you choose to do, this paper has
given you the tools to make a more educated decision for you and your baby.
References
Armstrong, B. G., McDonald, A. D., & Sloan, M. (1992). Cigarette, alcohol, and coffee
consumption and spontaneous abortion. American Journal of Public Health,
82(1), 85-87.
Bech, B. H., Nohr, E. A., Vaeth, M., Henriksen, T. B., & Olsen, J. (2005). Coffee and
fetal death: a cohort study with prospective data. American Journal of
Epidemiology, 162(10), 983-990.
Clausson, B., Granath, F., Ekbom, A., Lundgren, S., Nordmark, A., Signorello, L., et al.
(2002). Effect of Caffeine Exposure during Pregnancy on Birth Weight and
Gestational Age. American Journal of Epidemiology, 155(5), 429-436.
Cnattingius, S., Signorello, L. B., Granath, F., Anneren, G., Clausson, B., Ekbom, A., et
al. (2000). Caffeine intake and the risk of first-trimester spontaneous abortion.
The New England Journal of Medicine, 343(25), 1839-1844.
Fenster, L., Eskenazi, B., Windham, G., & Swan, S. H. (1991). Caffeine consumption
during pregnancy and fetal growth. American Journal of Public Health, 81(4),
458-461.
Heller, J. (1987), What Do We Know About the Risks of Caffeine Consumption in
Pregnancy?. British Journal of Addiction, 82: 885–889. doi: 10.1111/j.13600443.1987.tb03908.x
Howards, P. P., Hertz-Picciotto, I., Bech, B. H., Nohr, E. A., Anderson, A. N., Poole, C.,
et al. (2012). Spontaneous abortion and a diet drug containing caffeine and
ephedrine: a study within the Danish National Birth Cohort. PLoS ONE, 7(11), 19.
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Mills JL, Holmes LB, Aarons JH, et al. Moderate caffeine use and the risk of
spontaneous abortion and intrauterine growth retardation. JAMA.
1993;269(5):593-597. doi:10.1001/jama.1993.03500050071028.
Vik, T., Bakketeig, L. S., Trygg, K. U., Lund-Larsen, K., & Jacobsen, G. (2003). High
caffeine consumption in the third trimester of pregnancy: gender-specific effects
on fetal growth. Paediatric and perinatal epidemiology, 17, 324-331.
Vlajinac, H. D., Petrovic, R. R., Marinkovic, J. M., Sipetic, S. B., & Adanja, B. J. (1997).
Effect of caffeine intake during pregnancy on birth weight. American Journal of
Epidemiology, 145(4), 335-338.
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Birth Weight
Article
Caffeine intake does
not increase the risk
of low birth weight
Caffeine intake
does increase the
risk of low birth
weight
Clausson et al.
Fenster et al.
Vlajinac et al.
Vik et al.
Mills et al.
Heller (Review)
Total
Miscarriage
Article
Bech et al.
Cnattingius
et al.
Howard
Armstrong
et al.
Mills et al.
Heller
(Review)
Total
4
Caffeine
intake does
not increase
the risk of
miscarriages
3
6
Caffeine intake does increase the risk
of miscarriages
4
Figure 1. This shows the results from 9 studies and 1 review that looked at the
effects of caffeine on miscarriages or low birth weight and their overall conclusion.
From this table, you can see that the majority of studies concluded that caffeine has
an effect on miscarriages and birth weight.
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Adjusted Odds Ratio
3
2.5
2
1.5
1
0.5
0
✔ ✔
Vik et al.
Fenester et al.
Birth Weight (lbs)
Effect of Caffeine on Birth
Weight
Effect of Caffeine on Birth
Weight
8.2
✖
8
7.8
7.6
7.4
7.2
7
✔
Clausson et
al.
Vlajinac et
al.
Caffeine Consumption
Caffeine Consumption
Caffeine's Effect on
Miscarriages
3
2
✔
✔
1
✖
Bech et
al.
Cnattingi
us et al.
Armstro
ng et al.
Mills et al. Percent of
Miscarriages
30
Percent of
Miscarriages
Adjusted Odds Ratio
Figure 2. These graphs show how different amounts of caffeine intake during pregnancy
affect birth weight. It is clear there are conflicting conclusions on whether caffeine has
an adverse effect on birth weight. A check indicates that the study found that caffeine has
an effect on birth weight. An X indicates that the study found that caffeine does not have
an effect on birth weight. Doses of caffeine varied between studies, but typically a low
dose was around 0-150 mg, a medium dose was around 150-300 mg, and a high dose was
around 300+ mg of caffeine.
10
Mills…
0
0
0
Caffeine Consumption
✖
20
1 to 100 to 200 to 300+
199 199 299
Caffeine Consumption (mg)
Figure 3. These graphs show the effects of caffeine consumption on miscarriages. It is
clear that data is conflicting. In these graphs 2 studies found that there was an effect
(indicated by a check) and 2 studies found no effect (indicated by an X). Doses of
caffeine varied between studies, but typically a low dose was around 0-150 mg, a
medium dose was around 150-300 mg, and a high dose was around 300+ mg of caffeine.
Mills et al. was kept in milligrams because the ranges of caffeine used were different than
other studies.
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Coffee
Coffee
Coffee
(Brewed)
(Boiled)
(Instant)
Tea
Soda
Cocoa
Chocolate
Meds
Clausson
et al.
Mills et al.
Howard et
al.
Armstrong
et al.
Bech et al.
Cnattingius
et al.
Fenester et
al.
Vlajinac et
al.
Vik et al.
Figure 4. This table shows the types of caffeine that each study monitored. Studies were grouped
into those that found that caffeine does not have an effect on miscarriages or low birth weight (in
red) and those that found it has an effect on miscarriage or low birth weight (in green). An “X”
and shaded in box indicates that the study monitored that type of caffeine.
Estimation of Caffeine (mg)
Estimation of Caffeine in Multiple Forms
140
120
No
Effect
Claussen et al.
100
80
Mills et al.
60
Cnattingius et al.
40
Fenester et al.
20
Vlajinac et al.
0
Coffee
(brewed)
tea
soda
Cocoa
Effect
Vik et al.
Form of Caffeine
Figure 5. This graph shows how various studies converted types of caffeine into
milligrams (mg). The brackets on the right indicate the conclusions the studies
reached. Studies estimated milligrams of caffeine differently among various forms.
15
Birth Weight
Study
Caffeine intake does not
increase the risk of low
birth weight
Clausson et al.
Prospective
Fenster et al.
Vlajinac et al.
Vik et al.
Mills et al.
Prospective
Total
Prospective:Retrospective 2:0
Miscarriage
Study
Bech et al.
Cnattingius et al.
Howard
Armstrong et al.
Mills et al.
Total
Prospective:Retrospective
Caffeine intake does not
increase the risk of low
miscarriage
Caffeine intake
does increase the
risk of low birth
weight
Retrospective
Retrospective
Retrospective
0:3
Caffeine intake
does increase
the risk of low
miscarriage
Prospective
Retrospective
Prospective
Retrospective
Prospective
2:1
1:2
Figure 6. This figure shows the two types of research designs used in the different
studies. Prospective design is more reliable and accurate than retrospective design.
The studies that concluded caffeine does not have an effect on miscarriages or birth
weight used the prospective design more than the studies that concluded caffeine
has an effect on miscarriages or birth weight.
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Caffeine
NO EFFECT
= 5000 people
Total=135,566
HAS EFFECT
= 5000 people
Total=93,836
Figure 7. This is a graphical explanation of Figure 8 and shows the total number of
people used in the studies that found caffeine does not have an effect on miscarriages
or birth weight (on the left), and the studies that found that caffeine has an effect on
miscarriages or birth weight (on the right).
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Study
Clausson et al.
Fenster et al.
Vlajinac et al.
Vik et al.
Mills et al.
Bech et al.
Cnattingius et al.
Howard et al.
Armstrong et al.
Mills et al.
Total # of People
Caffeine intake DOES
NOT INCREASE the risk
of miscarriages or low birth
weight
953
Caffeine intake DOES
INCREASE the risk of
miscarriages or low birth
weight
2470
1011
358
431
88,482
1,515
97,903
35,848
431
135,566
93,836
Figure 8. This table shows the number of people in the individual studies and the number
of people in the combined studies based off conclusion.
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