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.

1

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

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

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.1360-

0443.1987.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), 1-

9.

11

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.

12

Birth WeightArticle 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 4 6

MiscarriageArticle Caffeine intake does

not increase the risk of miscarriages

Caffeine intake does increase the risk of miscarriages

Bech et al. ✔Cnattingius et al. ✔Howard ✖

Armstrong et al. ✖Mills et al. ✖Heller (Review) ✔

✔Total 3 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.

13

None Low Medium High7

7.27.47.67.8

88.2

Effect of Caffeine on Birth Weight

Clausson et al.Vlajinac et al.

Caffeine Consumption

Bir

th W

eigh

t (l

bs)

✖

✔

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.

0 1 to 199

100 to

199

200 to

299

300+05

10152025

Mills et al. Percent of Miscarriages

Mills ...

Caffeine Consumption (mg)

Per

cen

t of

Mis

carr

iage

s ✖

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

None Low

Mediu

m

High

00.5

11.5

22.5

3

Effect of Caffeine on Birth Weight

Vik et al.

Fenester et al.

Caffeine Consumption

Ad

just

ed O

dd

s R

atio ✔✔

None Low Medium High0

1

2

3

Caffeine's Effect on Mis-carriages

Bech et al.

Cnat-tingius et al.

Caffeine Consumption

Ad

just

ed O

dd

s R

atio

✖

✔✔

✖

✔✔

14

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.

15

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.

16

Coffee (brewed)

tea soda Cocoa0

20

40

60

80

100

120

140Estimation of Caffeine in Multiple Forms

Claussen et al.

Mills et al.

Cnattingius et al.

Fenester et al.

Vlajinac et al.

Vik et al.

Form of Caffeine

Est

imat

ion

of C

affe

ine

(mg)

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.

Birth WeightStudy Caffeine intake does not

increase the risk of low birth weight

Caffeine intake does increase the risk of low birth weight

Clausson et al. ProspectiveFenster et al. RetrospectiveVlajinac et al. RetrospectiveVik et al. RetrospectiveMills et al. ProspectiveTotalProspective:Retrospective 2:0 0:3

No Effect

Effect

17

Miscarriage

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.

18

Study Caffeine intake does not increase the risk of low miscarriage

Caffeine intake does increase the risk of low miscarriage

Bech et al. ProspectiveCnattingius et al. RetrospectiveHoward ProspectiveArmstrong et al. RetrospectiveMills et al. ProspectiveTotalProspective:Retrospective

2:1 1:2

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).

19

Study Caffeine intake DOES NOT INCREASE the risk of miscarriages or low birth weight

Caffeine intake DOES INCREASE the risk of miscarriages or low birth weight

Clausson et al. 953Fenster et al. 2470Vlajinac et al. 1011Vik et al. 358Mills et al. 431Bech et al. 88,482Cnattingius et al. 1,515Howard et al. 97,903Armstrong et al. 35,848Mills et al. 431Total # of People 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.

20

21