Coffee and pregnancy: is it safe?

An espresso (30 ml): approximately 64 mg
A cup of:
Filter coffee (250 ml): approximately 113 mg
Decaffeinated coffee (150 ml): 1 to 3 mg
Black tea (150 ml): 40 to 70 mg (depending on brewing time)
Green tea (150 ml): 15 to 25 mg

It's the same molecule! The difference is that, in tea, it is bound to molecules called tannins, which means its release into the body is more gradual after drinking tea. 

A double-blind randomised controlled trial examining caffeine intake in 1,207 women evaluated data from 1,153 births [3]. An average intake of 182 mg per day did not affect the duration of pregnancy. Furthermore, a study assessed the effect of caffeine intake on delivery outcomes in 873 cases [4]. Again, no association was found with preterm births. Therefore, moderate caffeine intake does not appear to be a contributing factor to premature birth.

In adults, it is primarily metabolised by enzymes in the liver. However, as this enzyme system is not developed until childhood, the kidneys are the main route of excretion in newborns, with more than 80% of caffeine passing unchanged into the urine, compared with only 2 to 4% in adults [5].

Thus, the elimination of caffeine by the foetus depends on the maternal metabolism, the rate of which changes during pregnancy. Whilst caffeine clearance during the first trimester is comparable to that in a non-pregnant state, the rate slows by half and then by approximately one third during the second and third trimesters respectively, leading to greater foetal exposure to the caffeine ingested by the mother [5].

Caffeine stimulates the secretion of catecholamine stress hormones. Elevated levels of catecholamines have the potential to increase placental vasoconstriction and raise the foetal heart rate [8], leading to impaired oxygenation, which may be harmful to the baby's health [9].

The conclusions of meta-analyses of several studies show a significant increase in the risk of miscarriage associated with caffeine consumption. The percentage increase was 32% in one study [10] and 36% in another (for a daily intake of more than 150 mg, equivalent to one and a half cups of filter coffee, just over 2 cups of espresso, or 2 cups of black tea) [11]. 

It was also found that this effect was dose-dependent, with estimated increases of 7% [12] to 14% [13] for every 100 mg consumed per day during pregnancy (equivalent to one cup of filter coffee), and 19% [10] for every 150 mg.

These findings also apply to an increased risk of stillbirth, with a risk increase of between 9 and 19% for every additional 100 mg consumed [12], [13]. There is also a 7% increase in the risk of low birth weight and 10% for small size [13]. 

A recent 2021 study found that even a daily intake of 50 mg of caffeine (slightly less than the amount found in 1 shot of espresso, and more than twice that in a cup of filter coffee) was associated with a significant risk of giving birth to a baby with a low birth weight [15].

This study is the first to observe this effect; the results should therefore be interpreted with caution and require further research before this effect on health can be confirmed with certainty. 

Studies show that newborns of mothers who consumed caffeine display withdrawal symptoms. These include sleep disturbances, vomiting, increased heart rate, irregular breathing and heightened fine tremors, similar to neonatal narcotic abstinence syndrome. These findings are significant compared with infants whose mothers did not consume caffeine [16].

A study in mice indicated that maternal coffee consumption may impair the mobilisation of trace elements from liver stores early in life, and that this may lead to a reduction in haemoglobin synthesis [6].

​​A modelling study was carried out using data from 1,083 mother-child pairs drawn from a population-based birth cohort in France, followed from pregnancy up to the age of 5.5 years [7]. 

Consumption was common in the sample: 91% of pregnant women consumed caffeine, and 12% consumed more than 200 mg/day. Children of mothers who consumed more than 200 mg/day were more likely to have a borderline or below-average IQ compared with children of mothers who consumed less than 100 mg/day (13.5% vs 7.3%).

This direct effect on the brain was also observed in a study that identified behavioural difficulties in children whose mothers had a high caffeine intake [18].

In one analysis, they showed that in utero exposure is broadly associated with an 87% increased risk of childhood obesity. This association demonstrated a dose-response relationship, with the risk doubling between a maternal daily intake of <150 mg per day versus an intake of ⩾150 mg per day during pregnancy [17]. 

Please note, this is the first time a link has been made with obesity. Further research is needed before it can be confirmed with certainty that a link between the two exists.