They have anti-inflammatory properties and they also block bacterial growth that is associated with between 10 to 30 per cent of preterm births. The bacteria, F nucleatum, travel from the mouth to the uterus, where they cause infection in the placenta.
The oils fight the infection that is also responsible for miscarriage and still births, say researchers at the Columbia University Irving Medical Centre.
Everyone has the bacteria in their mouth, but hormonal changes in women who are pregnant can cause gums to bleed, which allows them to spread through the body.
The oils are already recommended for pregnant women, but the researchers estimate the amount should be higher than the daily recommended dose to protect against infection.