There is considerable evidence (see previous article summary, for example) that maternal cigarette smoking around conception and during pregnancy is associated with increased risk of miscarriage and fetal growth restriction (FGR) resulting in low birth weight.
This study measured and compared the distribution of cotinine (nicotine metabolite) in fetal fluids during the first half of pregnancy in passive and active smokers.
The results provide direct proof that cotinine accumulates in the early fetal circulation and fluids in measurable concentrations. Although the placenta might be a relatively effective barrier, protecting the newborn at the end of pregnancy, the fetus is exposed early in the pregnancy, when the placenta seems to be more permeable. The fetus appears to be susceptible to chronic toxic exposure from both the mother's active smoking and the secondhand smoke in her environment.
Conclusions: Because cotinine accumulates in the fetal fluids as early as seven weeks' gestation in both active and passive smokers, women should be advised to give up smoking from conception and avoid environmental tobacco smoke exposure."
Jauniaux E, Gulbis B, Acharaya G, et al. Maternal tobacco exposure and cotinine levels in fetal fluids in the first half of pregnancy, Obstetrics and Gynecology, Jan. 1999; vol. 93, no. 1, pp25-29.