I used to teach college courses covering the topic. The text books said that smoking causes underweight premature babies. Because of this babies of smoking mothers are more likely to have birth defects. With alcohol, two drinks a day was considered safe, but with tobacco, there was no safe threshold. I thought this was rather strange. You smoke one cigarette while pregnant and you are more likely to have birth defects? Even for a hard core health fanatic that is difficult to believe.
Here is what was found in the data. Babies of smoking mothers average weight was 3232 grams (7.1 lbs.). Babies of non-smoking mothers averaged 3398 grams (7.5 lbs.). That is about a half pound difference and it is statistically significant. Seven pounds is a good healthy birth weight that does not set off any alarms. Babies are considered underweight if they are less than 2270 grams (5 lbs.). 4.5% of smoking mothers babies were underweight and 3.3% of non-smoking mothers babies were underweight. This difference is not significant. There is no indication here of a health risk from smoking based on weight.
The other risk factor is length of term. Normal gestation is 253 days. 4% of smoking mothers did not go to term and 7.8% of non-smoking mothers did not go to term. Smoking mothers did better than non-smoking mothers but the difference was not significant. There was obviously no risk from reduced term for smoking mothers.
Because the non-smoking mothers had heavier babies one would expect more C-Sections from the non-smoking mothers. There were about 20% more. This is significant at the .05 level but not the .01 level so you could argue the significance either way depending on your bias. The data here is limited because only 5% of pregnant women smoked but the trend for smoking mothers was toward less babies retained in the hospital, less C-Sections, insignificantly fewer pre-term deliveries and an insignificant increase in clinically underweight babies.
This data can be explained by assuming that when pregnant women are stressed, they self medicate to relieve the stress. Non-smoking women tend to eat more causing the baby to be larger and more difficult to deliver. This can also cause other problems. Smoking women tend to light up when under stress. This is less harmful to the baby than over-eating. For this reason smoking mothers tended to have better outcomes for baby and mother. They also cost less for the insurance company.
You might be interested in knowing that this information was not used. I was told that the medical insurance business is highly regulated by the government. The company was not allowed to tell the truth about these results even though it was better for the insurance company and for the patients.
I do not think these results suggest that women should start smoking when they get pregnant. I do think it indicates that it is very poor practice to try to get smoking mothers to stop smoking when they get pregnant.
I have a Ph.D. in experimental psychology and have worked in both research and teaching. I am a health nut and do not endorse smoking or care to be around people smoking. I was shocked by these results. My bias if any is certainly against these results. However I think it is horrible to withhold information form people and intentionally give them bad advice to advance a political agenda.