The Budget brought good news for working mums-to-be. Paid maternity leave has been extended to 18 weeks, with a further extension of non-paid leave to eight weeks.

Most women work through pregnancy without any problems, but for some it can be difficult, depending on the type of work they do and on any pregnancy complications. However, pregnant workers are protected by specific legislation, The Safety, Health and Welfare at Work (Pregnant Employees) Act 1994.

What are the circumstances which require a pregnant woman to take sick leave? In the early stages, a threatened miscarriage certainly means mandatory bed rest. It may be 14 weeks into a pregnancy before the threat of losing a baby recedes and an employee can return to work. Even then, the need for lifting and other manual tasks should be reviewed by the woman's employer before she returns to her previous job.

Pregnancy sickness (morning sickness can be a misnomer as many mothers-to-be experience nausea and vomiting in the evenings) is another common reason for time off work. This usually clears up at around 12 to 14 weeks, but it can last well into the middle trimester. When it is severe, a pregnant woman simply cannot work because of the nausea and the light-headedness brought on by not eating. Later in pregnancy, profound tiredness can be a problem. This may occur simply because of the increasing demands of a growing baby or it may be due to anaemia, in which a mother's blood count drops because her baby is commandeering all available iron supplies. Either way, tiredness is a legitimate reason for taking time off in the last weeks before confinement.

High blood pressure is another reason for ceasing work late in pregnancy. Although more common in first-time pregnancies, pre-eclampsia can occur in subsequent ones as well. If there is a danger of this, bed rest is a must and hospital admission may sometimes be necessary, usually until the baby is safely delivered.

A pregnancy-related bleed is another reason for stopping work at an earlier stage than usual. A low-lying placenta can be the cause of such bleeds and work should cease if a more severe haemorrhage is to be avoided.

As to the nature of work itself, there are a number of situations which pregnant employees should not be exposed to. Laboratory workers must not handle the rubella virus (unless adequately immunised) or the toxoplasma bug, both of which can harm a foetus. Lead and lead substances can also harm unborn babies, as can an enclosed, pressurised work environment where physical injury is a risk. The 1994 legislation covers women who are pregnant, those who have given birth within the past 18 weeks and those who are breast-feeding within 26 weeks of the date of confinement.

Employers are bound to identify all the pregnancy hazards in a workplace and to provide employees with a written risk assessment of these hazards. They must also detail the measures which are being taken to protect pregnant women from any risk to their health and safety.

A risk assessment identifies specific dangers to pregnant employees and, if it is not possible to remove these dangers from the work process, employers have a number of specific obligations.

Most importantly, they must transfer at-risk employees to other safer work. They may also have to adjust working hours or conditions temporarily. For example, night workers who work for three hours or more between 11 p.m. and 6 a.m. must be offered day work instead. If other suitable work or times cannot be arranged, employers are obliged to offer a special category of paid leave to the employee. This is called health and safety leave, and it usually lasts until maternity leave starts.

There must also be provision for pregnant women and nursing mothers to be able to lie down and rest in appropriate conditions.

If you are worried about exposure to chemicals in the workplace, you should ask for the "material safety data sheets" (MSDS) of the substances being used. The following risk phrases mean that you should not be exposed to the substance while pregnant:

R40 - possible risk of irreversible effect;

R45 - may cause cancer;

R46 - may cause inheritable genetic damage;

R61 - may cause harm to an unborn child;

R64 - may cause harm to breast-fed babies.

Excessive heat and cold and inappropriate noise levels represent conditions which are unsuitable for all employees, whether pregnant or not, but which must be specifically assessed as part of the pregnant employee regulations.

I am often asked about the effect of standing up during the latter part of pregnancy. My answer is that prolonged periods of standing should be avoided. If a woman has varicose veins or swollen ankles - both conditions are associated with pregnancy - then she should not stand for a full work shift. In my experience, most jobs can be altered slightly to allow alternate periods of standing and sitting. This is the best solution for the last 12 to 14 weeks of pregnancy.

Work-related pregnancy hazards are eminently preventable. As soon as you discover you are pregnant, inform your employer so that an individual risk assessment can be carried out for you.

Dr Muiris Houston, Medical Correspondent, can be contacted at mhouston@irish-times.ie or leave messages on tel. 01- 6707711, ext 8511. He regrets he cannot reply to individual medical problems.