Almost 7pc of mums-to-be smoking 10 cigs every day
The risk of babies being born preterm or at smaller birthweight has not fallen in one of Dublin's major maternity hospitals - because of the high levels of heavy smoking among mothers-to-be.
A study of births at the Coombe Maternity Hospital between 2009 and 2013 showed the number of moderate-to-heavy smokers went up from 3.9pc to 6.9pc.
These women were smoking 10 or more cigarettes a day while they were pregnant.
At the same time, overall smoking rates among the expectant mothers fell from 16.6pc to 12.6pc, the research, led by Dr Fionan Donohoe, revealed.
Expectant mothers who smoke are exposing their baby to harmful gases, such as carbon monoxide, and other damaging chemicals.
Smoking also leads to a greater risk of a range of complications, including premature birth and the baby being born underweight.
The researchers, who included Coombe obstetrician Professor Michael Turner, found that pregnant women in the study who smoked delivered smaller babies and were more likely to give birth early.
There are established risks to smoking during pregnancy, including stillbirth and having an underweight baby - who may face additional breathing, feeding and health problems.
The findings, presented to the annual scientific meeting of the Institute of Obstetricians and Gynaecologists, said that given the increase in the percentage of women who were moderate-to-heavy smokers, the risk of preterm delivery and lower birthweight has not been reduced.
The study follows a separate audit of services to help pregnant women stop smoking in the country's 19 maternity units.
It found there were wide variations in support available and overall they were inadequate compared to national and international recommendations.
Three of the 19 units did not ask expectant mothers on their first hospital visit whether they wanted to quit smoking during their pregnancy.
Just five said they routinely repeatedly asked a mother about smoking as the pregnancy progressed.
The authors, led by the UCD Centre for Human Reproduction and the Coombe Maternity Hospital, said the collection of information on whether the woman was smoking - and cessation advice - was often explored only at the first antenatal visit.
They stressed that improved services should be prioritised, because if a woman stops smoking in the first half of pregnancy there is a reduced risk of complications.
The national maternity strategy recommends that all maternity units should have tobacco-free campuses.
In Ireland, more than 5,500 smokers die each year from tobacco-related diseases.