Oestriol is one of three oestrogens naturally produced by women. Normally, levels in the body are very low, but during pregnancy, it is made in much higher amounts by the placenta. Oestriol levels increase throughout pregnancy and are highest just before birth. It is an indicator of the health of the unborn foetus because the chemical from which it is made comes exclusively from the adrenal glands of the baby. It causes growth of the uterus and increases its sensitivity to other pregnancy-related hormones, thus causing a gradual preparation for birth. Oestriol levels start to increase from week eight of pregnancy and scientists now think that labour begins when oestriol becomes the dominant hormone.
Oestriol is made by the placenta from a chemical that comes from the foetal adrenal gland called dehydroepiandrosterone sulphate. Oestriol levels increase throughout pregnancy because it stops other hormones from preventing dehydroepiandrostene sulphate being produced and thus allows more oestriol to be made.
A sudden surge in oestriol happens around three weeks before labour. If the surge comes early, this can suggest a premature birth.
Some hormone replacement therapy (HRT) preparations contain oestriol. Although the body removes oestriol much faster than other oestrogens, there are positives and negatives to its use in HRT.
In non-pregnant women, oestriol only exists at very low levels. Too little oestriol during pregnancy can indicate that there are problems with the baby, such as Down’s syndrome or problems with the placenta. Later in pregnancy, comparatively low oestriol indicates that labour may not come on its own, but will have to be induced.
Last reviewed: Jan 2015