More than one in ten pregnant women surveyed for new research were found to be obese.
The study found 43% were overweight - 13% obese and 2% morbidly so.
The UCD centre for Human Reproduction at Dublin's Coombe Hospital, which compiled the research, said the incidence of morbid obesity was high, increasing the risks to mother and baby.
Dr Nadine Farah said: "There is no national data on trends in maternal obesity but the incidence of morbid obesity in the CWIUH (Coombe Women and Infants University Hospital) is high compared with the limited amount of international data.
"Hospitals, midwives and obstetricians need to seriously consider the long-term impact of the rise in morbidly obese mothers and make provisions for appropriate equipment and resources to deal with this problem."
The research was conducted in 2007 among 5,824 women of all ages who had their body mass index calculated in their first trimester at their first antenatal visit to the Coombe.
The findings show that there was a much higher risk of complications for morbidly obese mothers, as 38.5% of the 116 had pregnancy-induced hypertension, compared with 9.8% in the normal BMI group.
There were also higher caesarean section rates - 45.3% compared with 14.4% - while 20% of morbidly obese mothers developed gestational diabetes.
The study also shows that maternal obesity in Ireland tended to be under-reported as mothers generally did not give an accurate BMI reading.
"Our research has shown that BMI calculations based on self-reporting leads to under-reporting of obesity in 5% of women," Dr Farah said.