The chemicals change a woman's body chemistry and give a false reading that a baby has a high chance of being born with Down's.
This puts women at high risk of miscarriage because they then have to undergo a further screening test for abnormality.
The findings, revealed by the Royal College of Obstetricians and Gynaecologists, are based on a study of 2,337 women in the first three months of normal pregnancies.
Those who had smoked just a few cigarettes a day came out with a high "false positive" score for an abnormal baby.
Professor Philip Steer, editor-in-chief of the British Journal of Obstetrics and Gynaecology, which published the study carried out at King Abdulaziz University in Saudi Arabia, said the findings were further proof of the damaging effects of smoking during pregnancy.
"Babies of mothers who smoke during pregnancy are small for gestational age and tend also to deliver earlier. This research indicates that smoking may also cause over-estimation of the risk of Down's, thus exposing the mother to a higher risk of an unnecessary amniocentesis."
All pregnant women in Britain are offered screening at the 15-week stage to check the foetus is developing normally. Any mother-to-be who is shown to have a high risk of a baby with Down's is then offered an amniocentesis, which involves a needle being inserted into the womb. Amniocentesis is only done in exceptional circumstances because there is a one in 100 chance of the test causing the mother to miscarry.
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