The research also showed that continued smoking compromised the cells' ability to repair damage, adding to negative effects.
And the scientists found new evidence that stopping smoking later during pregnancy greatly reduced the earlier damage, although it was best not to smoke at all.
The study results will appear in the international journal Human Pathology next month.
Lead author Dr Tania Slatter, of the Otago pathology department, said the findings had already been welcomed by some overseas scientists. Some researchers were ''kind of amazed that it hadn't been done before''.
Smoking in pregnancy had long been linked to lower birthweights and increased risk of serious complications, but the exact mechanisms had been unknown.
''We are particularly excited we now can give people more detail'' about some of the mechanisms involved.
Otago scientists had identified greatly increased rates of double-strand DNA breaks in smokers' placental cells. Such breaks were a severe form of DNA damage that could lead to cells becoming genetically unstable.
''Our study also showed a clear link between higher rates of double-strand breaks and lower birthweights and earlier delivery in mothers who smoked.''
The findings were also likely to help mothers to give up smoking during pregnancy, or for good.
It had previously been known that smoking during pregnancy had adverse effects on babies, but researchers previously could not say in detail ''it affects these cell types'', she said.
The Otago findings showed clear and specific damage, making it much harder for people to say ''it's not going to affect me''. Damage to cells in the placenta was likely to interfere with the transfer of nutrients from mother to unborn child and also with important hormone production.
Dr Slatter and her co-authors, including pathologist Dr Noelyn Hung and obstetrician Dr Celia Devenish, thanked the families who donated samples of their placentas and the midwives who supported their study participation. Funding came from the Healthcare Otago Charitable Trust.
- The findings emerged from an analysis of 236 placenta samples donated by women in the Otago Placenta Study after delivery. Of these women, 52 smoked throughout their pregnancy, 34 gave up smoking four weeks before delivery or earlier, and the other 150 were non-smokers.
