Taking aspirin throughout pregnancy could reduce the risk of the potentially dangerous condition pre-eclampsia, a major study suggests.
Pre-eclampsia can be symptomless
A University of Sydney team analysed data on more than 32,000 women for a study published in The Lancet.
The results suggested cases of pre-eclampsia, which is caused by a defect in the placenta, could fall by 10% if aspirin was taken widely.
Experts urged caution, given the small risks linked to long-term aspirin use.
Pre-eclampsia can trigger high blood pressure and kidney problems.
It affects approximately 7% of pregnancies, and, if uncontrolled, can increase the chances of both mother and baby dying.
Although up to 35% of premature births in the UK are connected to the condition, the precise reason it develops is unknown.
Pre-eclampsia is known to cause excessive blood clotting in the placenta, which supplies nutrients and oxygen to the foetus, and various experts have suggested that aspirin, which inhibits clotting, could counter this.
The latest research used information gathered by dozens of smaller research projects to try to come up with a reliable verdict on the risks and benefits of the treatment.
It found that taking "low-dose aspirin" during pregnancy reduced the risk not only of pre-eclampsia, but also premature birth, and of "poor pregnancy outcome" in general.
The main health risk found by other studies to be associated with aspirin is an increased chance of bleeding, which is potentially a serious issue within pregnancy and birth.
However, the study found no evidence that taking aspirin long term might be linked to bleeding problems at any stage, although the researchers said that their evidence was not strong enough to rule this out entirely.
Overall, they said, the potential benefits of taking the drug might outweigh the risks, particularly in women at higher risk of pre-eclampsia, such as overweight or older mothers, or those with a previous or family history of the condition.
They wrote: "From a public-health perspective, especially for populations with a high risk of pre-eclampsia, even these moderate benefits could make more widespread use of anti-platelet agents (aspirin) worthwhile."
US experts James Roberts and Janet Catov, from the University of Pittsburgh, said that while some women were so obviously at high risk that aspirin was justified, it was harder to balance whether the potential harm caused by aspirin was a price worth paying in pregnancies where pre-eclampsia was less likely.
They wrote: "Is treating 50 women to prevent one case of pre-eclampsia or one pre-term birth worthwhile?"
This was echoed by Mike Rich, chief executive of charity Action on Pre-Eclampsia, who said the study would help spread the message about the potential benefits of aspirin to a wider audience of doctors.
He said: "Under no circumstances should pregnant women self-medicate with aspirin. While this study suggests that aspirin can have benefits to women at high risk, the decision to use aspirin should only be made in consultation with your doctor."
A spokesman for the Royal College of Obstetricians and Gynaecologists added: "It is a moderate reduction of around 10% but given that pre-eclampsia is potentially serious for some women and their babies, this is an important finding.
"No single sub-group of women seems to benefit particularly from low-dose aspirin.
"The decision on whether to take it in pregnancy should be made following discussion between the woman and her obstetrician, taking into account her individual risk of developing the condition."