Maternal smoking’s negative effect on infant body size

Finnish researchers say that maternal smoking during pregnancy leads to reduced infant body length and head circumference, effects that remain unchanged even if a woman stops smoking during the first trimester. The findings, published in BMJ Open, underscore the need to counsel women about the importance of kicking the nicotine habit before they conceive.

Methods
The study from which the results came was register-based and involved the Maternal Exposure (MATEX) cohort from the Finnish Medical Birth Register. The authors analyzed 1.38 million singleton births without congenital anomalies from January 1, 1991 to December 31, 2016. Logistic regression was used to quantify the effect of maternal smoking, stratified by the maternal smoking status.

The authors aimed to analyze the effect of maternal smoking on body size and body proportions of newborns when the mother had smoked only during the first trimester, in comparison with continued smoking after the first trimester. They also evaluated how growth restriction associated with maternal smoking contributes to changes in body proportions.

Outcome measures were body proportions indicated by low brain-to-body ratio (defined as < 10^thpercentile); high ponderal index and high head-to-length ratio (defined as > 90^thpercentile); small body size for gestational age at birth (defined as weight, length or head circumference < 10^thpercentile) and preterm birth (PTB) (< 37 weeks) and low birth weight (2500 g). (Ponderal index is a measure of leanness calculated as a relationship between mass and height.)

Findings
The researchers found that compared to smoking only in the first trimester, continued smoking after the first trimester was associated with high ponderal index (OR 1.26; 95% CI 1.23 to 1.28) and high head-to-length ratio (OR 1.22, 1.19-1.26), corresponding with absolute risks of 22% and 19%, respectively). The effects were slightly lower in women who had quit smoking during the first trimester. Similar effects were seen for body size variables and low birth weight. PTB was not associated with smoking only during the first trimester. Stratification of the analysis by socioeconomic status showed that neither lifestyle factors nor health behaviors statistically significantly influenced the risk estimates.

Conclusions
The women who smoked in the study were younger and more likely to be nulliparous. Said the authors of their results, “The most important finding of our study is that although the risk for low birth weight decreases by smoking cessation during the first trimester, brain size and body length in relation to body weight seem not to catch up.” They posited that maternal smoking may affect cell proliferation during organogenesis in early prenatal development and noted that insults during that period have been shown to persist throughout life.