BMJ  2003;326:1269 (7 June), doi:10.1136/bmj.326.7401.1269

Letter

Consumption of coffee during pregnancy

Authors' reply

EDITOR—In a prospective study we found that coffee drinking during pregnancy was associated with an increased risk of stillbirth, but not with death in infancy.¹ Our results on stillbirth seemed to indicate a threshold effect around four to seven cups per day, but the risk estimate in women with the highest intake of coffee was based on only 11 stillbirths and we therefore had no possibility to explore further the relation between coffee and stillbirth in women drinking eight or more cups of coffee per day. Ludwig claims that a dose dependent association is the gold standard for establishing a causal relation. This is not correct. A monotonic, unidirectional dose-response curve is neither necessary nor sufficient for establishing a causal relation.²

Our result may represent a causal relationship or be due to other factors associated with coffee drinking and stillbirth. Sindos et al are particularly concerned about the possibility of unadjusted confounding, because no information was included about drug abuse. Drug abuse, especially of cocaine, among pregnant women, is fortunately a minor problem in Denmark and identified drug abusers are not included in our cohort.³ Drug abuse including cocaine is thus of little or no relevance as a confounder in our cohort.

Caffeine is regarded as the key component in studies of the potential effects of coffee. The path to death is usually multifactorial, composed of several component causes.² Caffeine may just be one causal component that in some settings together with other causal components leads to death but in other settings one or more of the other component causes are missing, or the time specific distribution of these component causes is different.

As for Jacobs's criticism of our statistical approach we believe that pairwise comparisons between each level of coffee intake and the reference category is technically appropriate, even if other approaches could have been chosen. Coffee is a commonly consumed stimulant and if our results represent a true causal relationship it has important health implications. However, the question on a causal relation is still open. Thus in an ongoing randomised study including 1000 pregnant women we are further exploring the effect of coffee with and without caffeine.

kiwi{at}perinatal.dk Perinatal Epidemiological Research Unit, Department of Obstetrics and Gynaecology, Aarhus University Hospital, DK-8200 Aarhus, Denmark

Department of Obstetrics and Gynaecology, Aarhus University Hospital

Department of Paediatrics, Aarhus University Hospital

Department of Epidemiology and Social Medicine, University of Aarhus, DK-8000 Aarhus C, Denmark

Danish Epidemiology Science Centre, University of Aarhus

References

1. Wisborg K, Kesmodel U, Bech BH, Hedegaard M, Henriksen TB. Maternal consumption of coffee during pregnancy and stillbirth and infant death in first year of life: prospective study. BMJ 2003;326: 420. (22 February.)[Abstract/Free Full Text]
2. Rothman KJ, Greenland S. Modern epidemiology. Philadelphia: Lippincott-Raven, 1998.
3. Kesmodel U, Kesmodel PS, Larsen A, Secher NJ. Use of alcohol and illicit drugs among pregnant Danish women, 1998. Scand J Public Health 2003;31: 5-11.[Abstract/Free Full Text]

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Relevant Article

Maternal consumption of coffee during pregnancy and stillbirth and infant death in first year of life: prospective study

Kirsten Wisborg, Ulrik Kesmodel, Bodil Hammer Bech, Morten Hedegaard, and Tine Brink Henriksen
BMJ 2003 326: 420. [Abstract] [Full Text] [PDF]