Maternal smoking does not affect fetal size as measured in the mid-second trimester

Cigarette smoking during pregnancy is causally related to birthweight, but we do not know whether fetal growth restriction is a continuous process or, if not, at what stage of pregnancy it affects weight gain.

A random sample of para 1 and 2 mothers, drawn from the population of pregnant women in Bergen and Trondheim, Norway, and Uppsala, Sweden, were examined by a detailed questionnaire concerning smoking habits, menstrual history and pregnancy dating, and subjected to morphometric sonography of their fetuses in or around week 17. Of the 547 study participants, 31.9% were smokers. Gestational age was primarily determined by the last menstrual period [LMP], except in those with irregular cycles, and in 30 cases (6.6% of those with regular cycles) in whom the biparietal diameter [BPD]-determined age deviated >14 days from the LMP-based date.

The analysis did not reveal any statistically significant differences between the fetuses of non-smokers, light smokers (0-9 cigarettes per day) and heavy (10+ cigarettes per day) smokers, regarding BPD, mean abdominal diameter [MAD] femur length [FL], and a 'body contour index': [BPD+FL]/MAD.

Tobacco-induced fetal growth restriction probably begins after gestational week 17.