Fear that their babies will be taken into care leads some pregnant drug abusers to avoid antenatal clinics, doctors at the National Drug Treatment Centre have reported.
Others are too busy raising money to finance their drug habit to attend routine ante-natal clinics, they write in an article in the Irish Medical Journal.
The authors, Dr John O'Connor and Dr D. Sloan, advocate methadone treatment for pregnant heroin abusers.
An attempt to detoxify and become completely drug-free during pregnancy "is regarded as even more dangerous than continued drug use, being more stressful for the foetus than the mother."
Previously published estimates suggest that each year more than 150 women who are addicted to heroin - often in combination with anti-depressants and sleeping pills - give birth in Irish hospitals, almost all in Dublin.
Heroin passes easily into the placenta, which supplies the foetus with nourishment.
Higher than normal rates of distress, stillbirth, premature birth and congenital defects have all been reported where pregnant women are abusing heroin, the authors say.
When born, the babies suffer withdrawal symptoms such as convulsions, irritability, disturbed sleep patterns and poor feeding.
Older children born to drug addicts have been found to suffer developmental delay and hyperactivity, but it is still unclear how much of this is due to drug use during pregnancy and how much is due to being raised in an environment where the mother continues to use drugs, the authors write.
Such an environment, they say, "is invariably a stressful one, with associated poverty, poor nutrition and concomitant use of cigarettes."
On average, pregnant drug abusers take a month longer than other women to book into antenatal clinics. This is partly because heroin abuse disrupts the menstrual cycle and the women may not realise that they are pregnant.
Many of the women claim the ante-natal clinics are too difficult to get to, either because of where they are located or because they presume administrative barriers will be placed in their way.
Some "have many other demands on their time and financing their drug habit may take precedence over routine ante-natal appointments, especially when perceived as not conferring them with any major benefits," the authors say.
"Some women fear that their child will be taken from them into care and present even later for their ante-natal visit."
Some are discovered to be addicted only when they begin to show withdrawal symptoms in the labour ward.
At high risk during pregnancy are foetuses whose mothers go through cycles of withdrawal and relapse during the pregnancy, and are multiple drug users.
The use of street drugs increases the exposure to hepatitis B or C and to HIV.
Methadone treatment, say the authors, can reduce illicit drug use and needle sharing. "Criminal activity may also be reduced. Maternal nutrition is usually improved."