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.
Oestriol causes growth of the womb (uterus) and increases its sensitivity to other pregnancy-related hormones, thus causing a gradual preparation for birth. Oestriol levels start to increase from the 8th week of pregnancy, and scientists now think that labour begins when oestriol becomes the dominant hormone in the pregnant woman. It can be measured in the blood during the 15th and 20th weeks of pregnancy as a marker that helps detect birth defects in the developing foetus.
A chemical called dehydroepiandrosterone sulphate (DHEAS) is first produced by the foetus’ adrenal glands and is then transported to the foetal liver and made into 16a-hydroxy-DHEAS, which is a naturally occurring steroid. The 16a-hydroxy-DHEAS is then transported to the placenta where it is then made into oestriol. For most of pregnancy, the oestriol made is bound to other chemicals thereby preventing the oestriol from exerting any biological effects.
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 also contain oestriol, given to older women in the peri-menopause, or the menopause. Although the body removes oestriol much faster than other oestrogens, there are positives and negatives to its use in HRT. See the articles on menopause and What is HRT? for more information.
In non-pregnant women, oestriol only exists at very low levels. Too little unbound oestriol during pregnancy can indicate that there are problems with the baby, such as Down syndrome, or other 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 with artificial methods.
In some pregnant women with autoimmune diseases, it has been observed that their symptoms are not as severe during their pregnancy, especially the later stages of pregnancy when oestriol levels are highest. Based on these observations, some researchers are investigating whether oestriol is able to suppress the immune system and therefore if it could be used to relieve some of the symptoms of conditions such as multiple sclerosis and rheumatoid arthritis. Researchers are also looking into if oestriol can help menstrual related migraines, and post-natal depression, although this is still in the very early stages.
Last reviewed: Feb 2023