Oestriol is a hormone made during pregnancy that can be used to measure fetal health and predict when birth may happen.

Alternative names for oestriol

E3; estriol

What is oestriol?

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 fetus because the chemical from which it is made in the placenta, comes from another chemical which is first made in the baby’s adrenal glands and then gets altered in the liver of the baby before being finally converted to oestriol in the placenta. 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.

How is oestriol controlled?

Oestriol is made by the placenta from a chemical that comes from the fetus. The fetal adrenal glands first make a hormone called dehydroepiandrosterone sulphate (DHEAS). DHEAS is then transported to the fetal liver and made into 16a-hydroxy-DHEAS. The 16a-hydroxy-DHEAS is in turn transported to the placenta where it is then made into oestriol. For most of pregnancy, most of the oestriol made is bound to other chemicals thereby preventing the oestriol from exerting any biological effects.

What happens if I have too much oestriol?

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. See the articles on menopause and What is HRT? for more information.

What happens if I have too little oestriol?

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’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.

Future uses

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.

Last reviewed: Mar 2018