The plot of Mad Men contains the answer to this question. Madison Avenue spends much of the show working out how to deal with growing evidence of the harms of smoking and the impact of the Surgeon General's Report, first produced in 1964.

The report highlighted the deleterious health consequences of tobacco use. Smoking and Health: Report of the Advisory Committee to the Surgeon General held cigarette smoking responsible for a 70 percent increase in the mortality rate of smokers over non-smokers. The report estimated that average smokers had a nine- to ten-fold risk of developing lung cancer compared to non-smokers: heavy smokers had at least a twenty-fold risk. The risk rose with the duration of smoking and diminished with the cessation of smoking. The report also named smoking as the most important cause of chronic bronchitis and pointed to a correlation between smoking and emphysema, and smoking and coronary heart disease. It noted that smoking during pregnancy reduced the average weight of newborns. On one issue the committee hedged: nicotine addiction. It insisted that the "tobacco habit should be characterized as an habituation rather than an addiction," in part because the addictive properties of nicotine were not yet fully understood, in part because of differences over the meaning of addiction.

The 1964 report on smoking and health had an impact on public attitudes and policy. A Gallup Survey conducted in 1958 found that only 44 percent of Americans believed smoking caused cancer, while 78 percent believed so by 1968. In the course of a decade, it had become common knowledge that smoking damaged health, and mounting evidence of health risks gave Terry's 1964 report public resonance. Yet, while the report proclaimed that "cigarette smoking is a health hazard of sufficient importance in the United States to warrant appropriate remedial action," it remained silent on concrete remedies. That challenge fell to politicians. In 1965, Congress required all cigarette packages distributed in the United States to carry a health warning, and since 1970 this warning is made in the name of the Surgeon General. In 1969, cigarette advertising on television and radio was banned, effective September 1970.

It's not a taboo (which suggests an irrational belief), it's based on evidence of harm to the unborn child. There are associations with low birth weight, still birth, prematurity and increased long-term risk from things like heart disease and diabetes.

We didn't have the research tools to identify these harms until the big postwar showdown with the tobacco companies and the work of, especially, Richard Doll and colleagues.

At the end of the Second World War, Britain had the highest incidence of lung cancer in the world – and no one knew why. Doll himself originally thought the increase might be due to occupational factors, or to the tarring of roads, as there were known carcinogens in tar.

By the end of 1950, studies from different countries had linked smoking and lung cancer, the best being those by Doll & Hill and by Wynder & Graham. Doll and his mentor Bradford Hill went on to show that smoking caused not only lung cancer but also many other diseases. Even with solid science, it was many frustrating years before the medical profession, politicians and the public really accepted the evidence.

A part of the reason for those "many frustrating years" is that the health consequences of things like smoking and drinking are not as obvious or immediate as, say, jumping off a tall building without a safety net. Before WWII we didn't have the research methods required to address questions like this, where it is not possible to do experiments. We cannot randomise ten year olds to become lifetime smokers or lifetime non-smokers, so we cannot directly prove causation. Cigarettes were once promoted by doctors and marketed as aids to health and one of the arguments the tobacco companies used to deflect from the growing evidence of harm was that people who were prone to lung disease were more likely to smoke because it helped them.

The evidence on drinking in pregnancy came along a little later than that for smoking, but using the same observational methods. Those data are a little less clear cut and (very) moderate drinking is now seen by some medics as probably not very harmful, but these questions are really very difficult to answer well. We can experiment on women during pregnancy a little more easily than we can on ten year olds for a lifetime but the ethics of it are still very, very difficult and it is rarely done.