Physical activity (PA) in pregnancy is important for maternal and possibly offspring health. To study the early origins of lung function we aimed to determine whether PA in the first half of pregnancy is associated with lung function in healthy 3-month-old infants.

From the general population-based Preventing Atopic Dermatitis and Allergies in Children birth cohort recruiting infants antenatally in Norway and Sweden, all 812 infants (48.8% girls) with available tidal flow–volume measures in the awake state at 3 months of age and mid-pregnancy data on PA were included. PA was self-reported by the mothers and, based on intensity, we categorised them as active or inactive during pregnancy. Furthermore, we defined active mothers as fairly or highly active. The main outcome was a ratio of time to peak tidal expiratory flow to expiratory time (t_PTEF/t_E) <0.25. Associations were analysed by logistic regression, adjusting for maternal age, education, parity, pre-pregnancy body mass index, in utero nicotine exposure and parental atopy.

The mean±sd t_PTEF/t_E was 0.391±0.08 and did not differ significantly according to maternal PA level in pregnancy. The 290 infants of inactive mothers had higher odds of having t_PTEF/t_E <0.25 compared to infants of all active mothers (OR 2.07, 95% CI 1.13–3.82; p=0.019) and compared to infants (n=224) of fairly active (OR 2.83, 95% CI 1.26–7.24; p=0.018) but not highly active mothers (n=298).

Based on self-reported maternal PA in the first half of pregnancy, 3-month-old infants of inactive compared to active mothers had higher odds of a low t_PTEF/t_E.