Hi, Katia!
Thanks for your comment and thoughtful questions! I can’t answer the smoking question with certainty, but I have read that the body can recover fairly quickly after smoking cessation. If a mother is worried it will affect the fetus, I’d advise her to counteract it with a high-nutrient, plant-based diet. As far as eating fish during pregnancy goes, there are certainly some types of fish that contain lower levels of mercury and other environmental toxins. Most health professionals don’t want to advise patients to cut out fish entirely because compliance would be very low, so they offer a list of fish that are “OK” to eat. These days, all fish contain toxins. There’s nothing in fish that you can’t get from a plant-based diet, so why risk it and eat fish at all? Results of nutrition studies on pregnancy outcomes need to be taken with a grain of salt because it’s very difficult to isolate variables. It’s possible that the women in the study who ate more fish were also eating a better overall diet, which led to better outcomes. Here’s a good video on the subject: http://nutritionfacts.org/video/how-long-to-detox-from-fish-before-pregnancy/
I hope that helps!

Thank you for your wonderful tips! I agree that the health state of every women prior to conception will influence their child’s future health too. Do you think if somebody give up smoking 6 month before pregnancy, it will also have a bad influence on child’s future health? Regarding the placenta barrier you mentioned, I want to add that placenta filters out many substances which can be harmful to the fetus, but it cannot filter out all toxins. You also recommended to avoid any fish. I found that you can eat fish but need to choose it very carefully. Some studies show that eating fish during pregnancy may help prevent preterm birth and low birth weight. What do you think?

Thank you again!