Paradoxically, some of the hazards of travel during pregnancy have increased in recent years. This is partly due to the continuing spread of drug-resistant malaria, and also arises from the fact that countries with poor medical care have become increasingly accessible to the adventurous traveller.

Good antenatal care has brought about a dramatic reduction in the complications of pregnancy, and travel has become almost too easy - it is often taken for granted. Perhaps the first hazard that the pregnant woman faces is a psychological one; pregnancy is not the ideal time for adventurous travel, but there is a widespread belief that travel to any country should be possible and that the fact of pregnancy should not be allowed to get in the way.

The early weeks of pregnancy are an important time to be at one's home base. It is necessary to begin planning antenatal care, and to arrange routine blood tests and ultrasound scans. Morning sickness is common, and as a result many women have no particular interest in travel at this stage. Early pregnancy is also a time when miscarriage is relatively more common. Travel itself does not increase the risk of miscarriage, but the consequences in a country where medical facilities are poor could be serious. If bleeding is severe, blood transfusion may be necessary. In many poor countries the risk of AIDS from unscreened blood transfusions is high, and facilities for surgery (including supplies of sterile medical instruments) may be difficult to obtain. Poor medical treatment may have serious consequences for future pregnancies.

Towards the later stages of pregnancy, premature delivery becomes a possibility. It is not generally feasible to predict which pregnancies are at risk. Survival of a premature baby depends upon immediate access to sophisticated neonatal intensive care facilities, and the greater the prematurity the more important this becomes. Even when such facilities are available they may be extremely expensive, and the cost of neonatal intensive care may not be covered by travel insurance. Severely premature babies may not be able to travel for several weeks, adding further to the cost. Facilities for skilled medical care during delivery, surgical facilities and access to adequate blood transfusion facilities may again be a problem.

Aeroplanes do not make good delivery suites, and while air travel does not in itself induce labour, long flights should be avoided during late pregnancy; in any case, most airlines do not accept passengers beyond the 32nd week of pregnancy.

Chief hazards

Two direct hazards of travel deserve mention. The first is the fact that there is an increased tendency for blood to clot in the veins of the legs - deep vein thrombosis. This tendency is accentuated by dehydration and prolonged immobility, both of which are common during long air journeys. The preventive measures are simple: drink plenty of fluids, stand up and walk around the aircraft cabin at least every two hours during a flight. The same applies to travel by road: take a rest and stretch your legs at least every one to two hours on a long journey.

The second hazard has received much attention over the last two years and relates to exposure to radiation. It has long been known that exposure to cosmic radiation at normal flying altitudes (10,500m) is more than 100 times greater than at ground level. There has been increasing concern about the effect of low-dose radiation and calculations show that it is possible for frequent flyers to build up a significant radiation exposure. Solar flares - bursts of energy on the surface of the sun - account for periodic increases in such exposure, and occur in unpredictable patterns. The radiation exposure for a return trip between London and New York is roughly equivalent to the exposure from a single chest x- ray (0.1 milliSievert); a return flight between London and Los Angeles would clock up 0.16 mSv. Calculations on the extent of harm associated with radiation exposure are generally based on exposure to much larger doses - such as those that occurred at Hiroshima.

It is difficult to be sure how such results extrapolate to lower doses and it is conceivable that low doses may be relatively more harmful. It is also difficult to document the effects and to know whether subtle changes such as differences in intelligence or minor defects can be attributed to such exposure rather than nature. For this reason, it has been suggested that pregnant women should avoid unnecessary long-distance flights during the early, most vulnerable stages of pregnancy.

Vaccinations involving a live virus should be avoided during pregnancy: these include the oral polio vaccine, and the vaccines for measles, rubella and yellow fever. If a yellow fever vaccination certificate is necessary for travel, a medical certificate can circumvent the requirement. Protection against polio can be provided using a killed, injectable vaccine. Vaccines that commonly cause a fever, such as the one giving protection against diphtheria and the injectable typhoid vaccine, should be avoided during pregnancy and the BCG vaccine should not be given.

Drug-resistant malaria continues to spread, and the particular problem with malaria in pregnancy is that malaria attacks tend to be considerably more severe. Both mother and foetus may be at very high risk. There are now relatively few parts of the world where chloroquine and paludrine - the two safest drugs for use in pregnancy - provide reliable protection. Mefloquine (Lariam) is a newer antimalarial drug that is now widely used for travellers to resistant areas, but there are parts of the world - especially in the region of the 'Golden Triangle' of South-East Asia - where resistance to mefloquine is common. Mefloquine has not been in use long enough for a clear picture to have emerged regarding its safety for use in pregnancy; there is no objective evidence of a risk, but caution is still advisable, especially during early pregnancy.

In addition to medication, insect repellents and other anti-insect measures (mosquito netting, suitable clothing, insecticide sprays, etc.) should also be used assiduously to reduce the number of mosquito bites. However, there is a strong case to be made for avoiding all unnecessary travel to malarial areas during pregnancy - particularly to areas with drug-resistant malaria.

Other tropical or infectious diseases tend to affect pregnancy only indirectly, such as causing dehydration or a high fever, both of which put the foetus at risk. Great care should be taken to avoid diseases such as dengue fever (by use of anti-insect measures) and to observe careful food and water hygiene measures.

If travel during pregnancy is considered essential, it is important to find out as much as possible about local medical care - names and addresses of doctors, hospitals and facilities for neonatal intensive care, should anything go wrong. It is also important to take particular care to insure adequate insurance cover for both mother and child.

Experts consider that the most suitable time for an overseas trip during pregnancy - provided that there have been no complications or other problems - is after the majority of the ante-natal tests have been completed and the main risks of miscarriage are over, but before the foetus becomes viable and would need neonatal intensive care facilities if born prematurely. This period lies between the 18th and 24th week of pregnancy, though my own view is that high-risk countries should definitely be avoided throughout the pregnancy.