All pregnant women should be tested for smoking so that they can be given quitting advice if necessary, a health watchdog says.
The National Institute for Health and Clinical Excellence said carbon monoxide tests should be carried out on every expectant mother.
If implemented, every woman would have the breath test at her first ante-natal appointment.
Midwives criticised the test, saying it could make the women feel "guilty".
NICE said the guidelines were not aimed at penalising smokers but were designed to help women and their families give up smoking during and after pregnancy.
"During pregnancy, smoking puts the health of the women and her unborn baby at great risk both in the short and long-term, and small children who are exposed to second-hand smoke are more likely to suffer from respiratory problems," Professor Mike Kelly, Nice director of the centre of public health excellence, said.
"One of our recommendations is for midwives to encourage all pregnant women to have their carbon monoxide levels tested and discuss the results with them.
"This isn't to penalise them if they have been smoking, but instead will be a useful way to show women that both smoking and passive smoking can lead to having high levels of carbon monoxide in their systems."
The guidelines were welcomed by the Royal College of Midwives, but it urged "non-judgemental" support for women smokers.
RCM education and research manager Sue Macdonald said: "There appears to an emphasis on pregnant women, which is appropriate given the evidence. However, the key issue here for NICE is their emphasis on the monitor.
"It is crucial that health practitioners, including midwives, focus on being supportive rather than making women feeling guilty, or as though they may not be truthful.
"Use of the monitor has the potential to make women feel guilty and not engaged. We need to look at a range of individualised interventions for women that meet their needs and aspirations."
The cost of the monitors also raised concerns for the RCM, as well as safety, infection control, and "whether this is the best use of funds to address smoking cessation," she said.