Does caffeine cause uterine contractions?

Caffeine and Uterine Contractions

Caffeine consumption can stimulate uterine contractions, particularly in high doses, and is associated with increased risk of pregnancy complications including miscarriage, preterm labor, and reduced birth weight.

Effects of Caffeine on Uterine Activity

Caffeine acts as a central nervous system stimulant and can affect smooth muscle function throughout the body, including the uterus. The evidence suggests several important mechanisms:

• Caffeine is listed as a substance that may cause elevated blood pressure in the 2018 ACC/AHA hypertension guidelines 1, which can indirectly affect uterine contractility
• Studies in pregnant mice have demonstrated that caffeine exposure during early pregnancy impairs oviductal embryo transport and uterine receptivity 2
• Caffeine consumption is associated with increased risk of spontaneous abortion, with a dose-dependent relationship 3, 4

Pregnancy Risks Associated with Caffeine

The evidence shows clear associations between caffeine intake and adverse pregnancy outcomes:

• Moderate-to-heavy caffeine users (≥151 mg daily) have a 73% increased relative risk of miscarriage compared to non-users or light users 4
• A strong dose-response relationship exists between caffeine intake and late miscarriage/stillbirth, with odds ratios increasing to 5.1 for intakes of 300+ mg/day 3
• Caffeine intake during pregnancy is consistently associated with decreased birth weight and increased odds of small-for-gestational-age babies, even at levels below current recommended maximums 5

Clinical Recommendations

Based on the evidence of caffeine's effects on uterine activity and pregnancy outcomes:

• Pregnant women should limit caffeine intake to no more than 200 mg/day according to European Food Safety Authority guidelines 1
• Women experiencing preterm labor or at risk for preterm birth should be particularly cautious about caffeine consumption, as it may exacerbate uterine contractions 1
• Women with a history of previous miscarriage should be especially careful, as even light caffeine use (1-150 mg daily) was associated with increased risk for spontaneous abortion in this population 4

Caffeine Content in Common Sources

For context, caffeine content varies by source:

• Coffee: 50-300+ mg per 8 oz (237 mL) serving 1
• Brewed tea: 15-50 mg per 8 oz (237 mL) 1
• Decaffeinated coffee/tea: ~10 mg or less per 8 oz (237 mL) 1
• Carbonated soft drinks: variable, but a significant source of caffeine intake 1

Common Pitfalls and Caveats

• Individual sensitivity to caffeine varies significantly, similar to what is observed in human populations 2
• Many people underestimate their caffeine intake by not accounting for all sources (coffee, tea, chocolate, medications, energy drinks)
• The effects of caffeine may be more pronounced in women with pre-existing conditions like hypertension or those at risk for preterm labor
• Caffeine's effects may be compounded by other lifestyle factors like smoking, which should be assessed concurrently

In summary, while moderate caffeine consumption is generally considered safe for most non-pregnant adults, there is substantial evidence that caffeine can stimulate uterine contractions and is associated with adverse pregnancy outcomes in a dose-dependent manner.