Microchimerism and pregnancy: what happens?

Pregnancy is the main cause of natural microchimerism through bidirectional transplacental cellular exchange between the mother and the foetus. During a normal pregnancy, there is a reversible transfer of maternal, foetal, and placental cells. This occurs in all pregnancies and increases with gestational age.

A small number of cells from the mother persist in her child into adulthood (maternal microchimerism), while a small number of cells from previous pregnancies persist in the mother for many years (foetal microchimerism)[1].

Measuring the presence of the Y chromosome in women who have previously had male pregnancies is a practical way to assess microchimerism. Using this method, male cells of presumed foetal origin have been detected in 30 to 50% of women across a number of studies[2]. It is not necessary to have had a boy for foetal microchimerism to have occurred, but it is easier to observe in that case (thanks to the Y chromosome).

Cell exchange goes beyond mother and foetus

Microchimerism is thought not to be limited to the bidirectional exchange of maternal and foetal cells: cells from older siblings, or even cells from the maternal grandmother, can also be transferred to the foetus[3]! A cell exchange can also take place between twin foetuses in the uterus[4].

It is observed years after pregnancy

This transfer of foetal cells begins at the 4th or 5th week after fertilisation and continues throughout pregnancy. These cells can be detected up to 30 days in maternal blood post-partum[5].

It has been reported that male foetal progenitor cells have been detected in tissues up to 27 years after delivery[6].

Microchimerism is observed even in pregnancies not carried to term

To develop microchimerism, it is not necessary to continue a pregnancy and give birth to a child. Early terminations of pregnancy due to surgical abortion can deliver up to 500,000 foetal cells into the woman's bloodstream[7].

Male microchimerism has been observed in women who have never had a boy!

Male microchimerism has been found in one fifth of women who have never had a male birth. This can occur in several ways: early miscarriage of a male embryo, disappearance of a male twin, transfer of male cells from an older sibling via the maternal circulation, or a possibility yet to be explored — that of male DNA being transferred into the woman's circulation during sexual intercourse[8].

Some studies suggest that pregnancy could be a protective factor against certain cancers (breast, ovarian). For a long time, this effect was attributed to hormonal factors, but foetal microchimerism acquired during pregnancy may also contribute to this protective effect [9]. It could have a beneficial effect on the immune surveillance of malignant cells [10].

Around 80% of people with autoimmune conditions are women. Several hypotheses have been put forward to explain this sex-based difference, such as hormonal factors or stronger immune responses in women. 

Greater microchimerism is observed in women with scleroderma

Progressive systemic sclerosis, also known as scleroderma, is an autoimmune disease that primarily affects women in the post-partum years and bears a striking resemblance to graft-versus-host disease [17].

A study searched for male DNA in women with progressive systemic sclerosis and in healthy women who had given birth to at least one son. DNA levels were significantly higher in women with progressive systemic sclerosis than in healthy women. Some women with progressive systemic sclerosis who had given birth to a male child several decades earlier obtained results equivalent to those currently pregnant with a healthy male foetus [18].

Greater microchimerism is observed in women with Sjögren's syndrome

Sjögren's syndrome is an autoimmune disease characterised by dry mouth and dry eyes, which can also affect other parts of the body. It occurs more frequently in women over 40.

Foetal microchimerism was at the centre of a study involving 56 women with Sjögren's syndrome, of whom 42 had at least one male child. Male DNA was found in 29% of salivary gland samples and 22% of lung samples from patients with Sjögren's syndrome. None of these subjects had a history of blood transfusion. The PCR sequence of the male chromosome was not detected in control samples. Four subjects, aged over 60, had detectable foetal cells in their peripheral blood up to 27 years after delivery [19].

Microchimerism occurs during pregnancy, with an exchange of cells between the mother and the foetus. The mother can retain these cells in her body for a long time — up to 27 years after giving birth!

The impact of these cells on maternal health is not yet fully understood; some studies suggest they may have a protective effect against breast cancer, while others indicate they could promote the development of autoimmune conditions…

Either way, it's quite something to think that you carry your baby's DNA within you long after giving birth!