RESUMEN
STUDY QUESTION: How common is the use of herbal supplements during pregnancy and does it adversely affect the pregnancy outcome? SUMMARY ANSWER: The use of herbal products during pregnancy is very common and daily almond oil spreading is associated with preterm birth (PTB). WHAT IS KNOWN AND WHAT THIS PAPER ADDS: Herbal drugs are often promoted as 'natural' and 'safe' and such claims attract pregnant women. More than a quarter of Italian pregnant women consume herbs every day for at least 3 months during pregnancy. We raise an alert over the habit of daily almond oil spreading since it seems to be associated with PTB. DESIGN: A multicenter retrospective cohort study performed over a 15-month period. PARTICIPANTS AND SETTING: Seven hundred women interviewed within 3 days of childbirth, in three public hospitals in northern Italy. MAIN RESULTS AND ROLE OF CHANCE: One hundred and eighty-nine women were considered 'regular users', since they consumed herbs every day, for at least 3 months. Almond oil, chamomile and fennel were the most commonly used herbs. Both length of gestation and birthweight were affected by herb consumption. Almond oil users showed more pre-term birth (29 of 189) than non-users (51 of 511). After adjusting for multiple pregnancies, smoking, advanced age and drug intake, almond oil users maintained an increased risk to give birth <37th week (odds ratio = 2.09, 95% confidence interval: 1.08-4.08). BIAS, CONFOUNDING AND OTHER REASONS FOR CAUTION: The association between daily spreading of almond oil and PTB only raises a hypothesis that requires confirmation in larger trials devoted to this topic. The relatively small sample size did not allow the investigation of other adverse pregnancy outcomes in herb users. GENERALIZABILITY TO OTHER POPULATIONS: The population under investigation did not significantly differ from the general population attending the same hospitals. STUDY FUNDING/COMPETING INTEREST(S): No conflict of interest exists. The study has been supported by a public grant from the University of Modena and Reggio Emilia. TRIAL REGISTRATION NUMBER: None.