More than half of unborn babies are exposed to increased levels of toxic air pollution, according to research.
There is nearly double the amount of pollution inside a car than there is outside and 57% of mothers travel more by car or taxi when they are in the later stages of pregnancy.
This means 2.6 million unborn babies may have been exposed to additional air pollution over the past six years, according to the study.
The Opinium research is being released to mark Clean Air Day, which is organised by environment charity Global Action Plan and will see at least 325 events taking place around the UK.
Chris Large, senior partner at Global Action Plan, said: "The rest of society needs to recognise that for pregnant women sometimes travelling in a car is the only option and that we all have a role to play in reducing the pollution affecting the next generation.
"In the same way we would consider smoking in front of pregnant women as harmful to the babies' health, so should we think of idling in our cars so causing unnecessary pollution as equally socially unacceptable."
Pregnant mother Celia Wyndham, a recruitment consultant in London, said the research was "worrying".
"Since I've been pregnant I've been more inclined to use taxis when previously I might have got a bus or Tube, especially when I'm out at night - you just get tired so easily.
"I didn't know a lot about air pollution and am really worried to learn that air pollution in a car can be worse than outside.
"I often collected my daughter from nursery in the car. Now, though, I'll probably walk more instead of travelling by car."
Global Action Plan says research shows that raised levels of nitrogen dioxide increases the risk of miscarriage by 16%.
The answer could be for more pregnant women to work from home but the Opinium research showed that only 16% of mothers were able to do this regularly in the third trimester.
For some roles it was simply not possible but another reason given was lack of support from employers.
Professor Jonathan Grigg, Professor of paediatric respiratory and environmental medicine at Queen Mary University of London, said: "Air pollution is having a devastating impact on the lives of everyone, particularly children, and embryos.
"These health impacts can include increased risk of developing pneumonia and asthma, and lower lung function growth.
"This just illustrates how vital it is that we all take action to reduce air pollution, and the need for more stringent standards and policies to address this issue."
Meanwhile, research from First Bus has shown that 57% of Britons are worried about levels of air pollution but 37% persist in driving themselves to work.
Some 87% are trying to lessen their impact on the environment but many still find using the car quicker and more convenient than other options.
Only 8% of those using their cars would consider using public transport and just 6% would car share.