Background Maternal smoking during pregnancy is linked to small for gestational age (SGA) birth (< 10th percentile). We examined inter-pregnancy changes in maternal smoking and the odds of SGA in the second child.

Methods A population-based cohort of prospectively collected anonymised antenatal and birth healthcare data (2003–2018) recorded by University Hospital Southampton, Hampshire, UK (SLOPE study) was used. The first two singleton pregnancies resulting in live births were analysed (n=15,525 women) using logistic regression to examine changes in self-reported maternal smoking in relation to whether the second child was SGA. We adjusted for maternal age, ethnicity, body mass index, educational attainment, employment status, partnership status, folate supplementation, infertility treatment, gestational diabetes and gestational hypertension at the first pregnancy (P1), length of the interpregnancy interval and previous SGA birth.

Results SGA occurred in 15.7% of all pregnancy 2 (P2) births in mothers smoking at both pregnancies, compared to 5.7% in never-smokers (reference group). Smoking at the start of both pregnancies was associated with higher odds of 2nd child SGA (adjusted Odds Ratio (aOR) 2.88 [95% CI 2.32, 3.56]). The aOR of 2nd child SGA were also higher in women who smoked only at the start of either P2 (2.02 [1.41, 2.89]) or P1 (1.52 [1.10, 2.09]). The aOR of 2nd child SGA were similar to never-smokers in those who quit when each pregnancy was confirmed (1.23 [0.81, 1.85]), smoked between pregnancies but quit up to P2 confirmation (0.82 [0.59, 1.15]), or quit by P1 confirmation and maintained cessation (0.91 [0.74, 1.11]). The odds of SGA birth for women with no previous SGA followed a similar pattern. Among women whose 1st baby was SGA (n=1,903), the aOR of recurrent SGA were higher in those smoking at the start of both pregnancies (2.62 [1.84, 3.72]), or at P2 only (1.82 [1.00. 3.30]). However, those who were P1 smokers and stopped by P2 were not more likely to have recurrent SGA (aOR 1.08 [0.62, 1.88]).

Conclusion Mothers who smoked at the start of either one or both of their first two pregnancies had increased odds of SGA birth compared to never-smokers. However, the odds of recurrent SGA with smoking in the first pregnancy and quitting at any point up to confirmation of the second pregnancy were similar to never-smokers. The time between pregnancies is an opportunity to intervene on modifiable risk factors such as smoking, particularly in those with previous history of SGA babies.

Funding Supported by an NIHR Southampton Biomedical Research Centre and University of Southampton Primary Care and Population Sciences PhD studentship (to EJT) and an Academy of Medical Sciences and Wellcome Trust grant; Grant number AMS_HOP001\1060 (to NAA).

Contributors NAA conceived the study idea and is PI of the SLOPE (Studying Lifecourse Obesity PrEdictors) study. NZ cleaned and managed the raw data. EJT analysed the data. EJT drafted the abstract. All authors contributed to the study design, interpreted the results, revised the abstract for content and approved the final version before submission.

Acknowledgements David Cable (Electronic Patient Records Implementation and Service Manager) and Florina Borca (Senior Information Analyst R&D) at University Hospital Southampton NHS Foundation Trust for support in accessing the data used in this study.