Background The association of maternal cigarette smoking during pregnancy with severe neonatal morbidity (SNM) is still inconclusive. We aimed to examine the associations of the timing and the intensity of maternal cigarette smoking with infant SNM in the USA.

Methods We used birth certificate data of 12 150 535 women aged 18–49 years who had live singleton births from the 2016–2019 US National Vital Statistics System. Women self-reported the daily number of cigarettes they consumed before pregnancy and in each trimester of pregnancy. Composite SNM was defined as one or more of the following complications: assisted ventilation immediately following delivery, assisted ventilation for >6 hours, neonatal intensive care unit admission, surfactant replacement therapy, suspected neonatal sepsis, and seizure.

Results Maternal cigarette smoking either before pregnancy or during any trimester of pregnancy significantly increased the risk of infant SNM, even at a very low intensity (ie, 1–2 cigarettes per day). For example, compared with women who did not smoke before pregnancy, the adjusted odds ratios and 95% confidence intervals (OR, 95% CI) of composite SNM in the newborn from women who smoked 1–2, 3–5, 6–9, 10–19, and ≥20 cigarettes per day before pregnancy were 1.16 (1.13 to 1.19), 1.22 (1.20 to 1.24), 1.26 (1.23 to 1.29), 1.27 (1.25 to 1.28), and 1.31 (1.30 to 1.33), respectively. Furthermore, smokers who stopped smoking during pregnancy still had a higher risk of composite SNM than never smokers before and throughout pregnancy.

Conclusions Maternal cigarette smoking before or during pregnancy increased the risk of infant SNM, even at a low dose of 1–2 cigarettes/day. Interventions should emphasise the detrimental effects of even light smoking before and during pregnancy.