Many pregnant women who smoke do not approach doctors for help to quit because they think they will be "judged", a poll for the NHS suggests.
Of 224 pregnant smokers questioned, 25% said they had not sought help because of worries about being criticised.
Some 40% said they had not admitted their habit to midwives or doctors.
The NHS is launching a new campaign to try to encourage the 17% of pregnant women in England who smoke to ask for help to give up their habit.
The Royal College of Midwives welcomed the initiative but warned women should not be stigmatised for smoking when pregnant.
Pregnant women who smoke do not automatically find it easy to stop smoking as soon as they become pregnant
Dr Miriam Stoppard
Smoking in pregnancy can reduce the amount of oxygen available to the developing foetus, and babies of smokers are more likely to be born with a lower birthweight.
The government has made a concerted effort to tackle smoking in pregnancy, spending £6m in 2002 to create specialists to advise women on the risks and help them to quit.
However, the survey, commissioned by the NHS Smokefree team, reveals that many women never even seek help in the first place.
Their concern is supported by other polls, which have found more than half of those surveyed are critical of pregnant smokers.
Dr Miriam Stoppard, who is leading the new campaign, said: "Pregnant women who smoke do not automatically find it easy to stop smoking as soon as they become pregnant.
"Pregnancy can be a particularly difficult time to stop smoking - and rather than stigmatising these women, we should be guiding them to their local NHS Stop Smoking Service for expert advice and support."
Lisa Fendall, one of the NHS specialist smoking in pregnancy advisors who covers the Rotherham area in Yorkshire, said: "We see lots of pregnant women who are worried about trying to stop smoking and feel they don't have the support or anyone to talk to about it."
She said that everything from group sessions to smoking helplines was available in her area.
A spokesman for the Royal College of Obstetricians and Gynaecologists said: "Healthy women have healthy babies who grow into healthy children.
"We are delighted to see such strong messages in an NHS public health campaign. These help remove some of the stigma identified by women as being barriers to change.
"However, we must go further. Apart from those already pregnant, we must also encourage and support women who are trying to conceive and those who have given birth to stop smoking."
Royal College of Midwives General Secretary Cathy Warwick, said: "Any steps to reduce smoking among pregnant women are to be applauded."
But she said: "In working to reduce smoking though, we must not point an accusing finger and stigmatize women who smoke during pregnancy.
"The focus should be on encouraging pregnant women to discuss the issue with their midwife who will offer help, support and guidance.
"Midwives and women can then work together to help the women stop, so that they and their babies can have a healthier future."
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