If a miscarriage happens during the first trimester of pregnancy (the first three months), it is usually due to problems with the foetus (the unborn baby).

If a miscarriage happens during the second trimester of pregnancy (between weeks 14 and 26), it is usually the result of an underlying health condition in the mother.

First trimester miscarriages

Most first trimester miscarriages are caused by problems with the chromosomes of the foetus.

Chromosomes are blocks of DNA. They contain a detailed set of instructions that control a wide range of factors, from how the cells of the body develop to what colour eyes a baby will have.

For a pregnancy to be successful, a foetus needs 46 chromosomes: 23 are from the father’s sperm and 23 are from the mother’s egg.

Sometimes, something can go wrong at the point of conception and the foetus receives too many or not enough chromosomes. The reasons for this are often unclear, but it means that the foetus will not be able to develop normally, resulting in a miscarriage.

Risk factors

Problems with chromosomes often happen by chance. However, a number of known risk factors increase the chances of these problems occurring.

Age

The most important risk factor for miscarriage is probably the age of the mother: