Exposure to maternal antibiotics during pregnancy may be associated with an increased risk of childhood asthma, according to recent study results published in Archives of Disease in Childhood.

Previous research has suggested there is an increased risk of adverse effects on offspring exposed to maternal antibiotics during pregnancy. In terms of asthma risk, the findings have been conflicting. Using data from the Danish National Birth Cohort (DNBC), investigators analyzed this relationship and whether establishing timing of antibiotic administration and mode of delivery were factors in increasing asthma risk.

The DNBC is a database that was established to examine the effect of prenatal and early life exposures. For this cohort study, pregnant women were recruited by their primary care physicians during a 6 year period; approximately 30% of all Danish pregnant women were included. Data including maternal smoking during pregnancy, prenatal body mass index (BMI,) maternal and paternal history of asthma, and socioeconomic status were collected from DNBC, as well as sex of child, maternal age at time of delivery, gestational age, and mode of delivery. Prenatal exposures and maternal factors were collected via 3 telephone interviews. An online questionnaire was given at 11 years to determine the child’s history of asthma.

A total of 32,651 participants (all singleton children) included in the analysis. Of these, 5522 (17%) of children studied were exposed to maternal antibiotics during pregnancy and a total of 4238 (13%) of children reported asthma during follow-up. After adjusting for confounders, antibiotic exposure during pregnancy was associated with an overall increased risk of childhood asthma (odds ratio [OR], 1.14; 95% CI 1.05-1.24). The overall absolute risk difference was 1.9% and the number needed to harm (NNH) was 53.

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Although there was no association between antibiotic exposure during the first trimester,  higher odds of asthma development were observed in individuals exposed in the second to third trimester (OR, 1.17; 95% CI, 1.06-1.28) compared with no exposure. Vaginal deliveries were also associated with higher odds of asthma (OR, 1.17; 95% CI 1.07-1.28) whereas there was no increased risk to those born via caesarean section (planned: OR; 0.95, 95% CI, 0.66-1.37; emergency: OR, 0.96; 95% CI, 0.73-1.28). Among vaginal births, the absolute risk difference was 2.2% and NNH was 45.

Limitations to this study included that antibiotic exposure and outcome data were obtained through maternal reporting, and recall may have been inaccurate with regard to timing of antibiotic exposure. Also, dosing of antibiotics was not available and as such a dose-response relationship could not be studied. Additionally, shared environmental and inherited risk factors were taken into account by adjusting for parental asthma and maternal smoking, but parental/ household smoking was not adjusted for and may be confounding. Similarly, maternal infection with or without antibiotic use was not studied and may affect the association.

“Antibiotic exposure in mid-to-late pregnancy was associated with childhood asthma by 11 years of age in vaginally but not caesarean section born children. Further studies should address the mechanisms underlying this epidemiological observation to identify possible interventions,” the study authors wrote. In addition, they noted that “[t]he profligate use of antibiotics in pregnancy should be balanced against the increasing evidence on adverse long-term health outcomes in the offspring, as well as broader concerns regarding antimicrobial resistance.”


Uldbjerg CS, Miller JE, Burgner D, Pedersen LH, Bech BH. Antibiotic exposure during pregnancy and childhood asthma: a national birth cohort study investigating timing of exposure and mode of delivery. Arch Dis Child. Published online February 9, 2021. doi:10.1136/archdischild-2020-319659