The Royal College of Midwives (RCM) has defended testing pregnant women for smoking.
Expectant mothers will be asked to take breath tests to prove if they are smoking during their pregnancy, under new NHS guidance.
An estimated 20% of women smoke while expecting, which is believed to lead to a low birth weight for their babies and cause complications in pregnancy and labour.
In a bid to lower the numbers, women should be tested for carbon monoxide found in cigarettes during antenatal appointments, said the National Institute for Health and Clinical Excellence (Nice), and given help to quit if levels are too high.
RCM chief executive Cathy Warwick said: "Midwives have a vital role to play in promoting public health and reducing smoking in pregnancy is extremely important. I visited a maternity unit this week and heard from fellow midwives just how helpful these tests can be in showing to women the potential damage that smoking can have on their baby.
"Of course, not all women will want to take this test. Any test which becomes routine must be offered along with comprehensive information and women must be able to opt out. Tests can help midwives educate women in the hope that they reduce their baby's exposure to cigarette smoke, but not all women will accept the test and it is only a partial solution."
She added that the NHS has too few midwives.
"The real solution here in cutting the prevalence of smoking in pregnancy is more midwives and more continuity of care from the same midwife," Mrs Warwick said.
"The shortage of midwives, particularly in England, means that often a midwife does not have as much time as she would want with each pregnant woman in her care and that women often see a different midwife at each visit.
"Nice is a listening organisation. They have asked for our views and we have given them. We would like them to make clear in any advice to women that midwives should offer the test but that ultimately the final decision must lie with the woman."