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The immune system’s aggressive natural killer cells – which normally kill cancer cells and infectious pathogens – also help nourish early fetuses, helping them grow.

This discovery was made by of the University of Science and Technology of China, in Hefei, and his team. Analysing natural killer cells from mice, they identified a subset that’s only produced in the uterus, and only during early pregnancy.

They named these “uterine NK cells”, and found that these cells produce large quantities of two proteins that are vital for growing fetuses.

One of these proteins, called pleiotrophin, drives the growth of blood vessels, bone, cartilage and brain fibres. The other protein, osteoglycin, orchestrates heart development and healthy growth of skin and eyes.

When the researchers examined womb tissue from 54 women, they found that those who had recently experienced miscarriages had fewer uterine NK cells than those who’d had successful pregnancies.

Possible treatment

Genetically engineering mice so that they could not make these killer cells caused them to produce fetuses that were only half the normal size. Giving extra uterine NK cells to these mice when pregnant, however, boosted the size of their offspring.

Tian’s team hopes that these cells can be used as a treatment for women who have recurrent miscarriages or underweight babies. It may be possible to inject the cells into a woman’s bloodstream, or deliver them vaginally.

“The concept that there are subtypes of uterine NK cells, and that they actively support development of the placenta and growth of the fetus is very exciting,” says , at the University of Cambridge. “This study adds considerably to our knowledge of the fascinating foetal-maternal dialogue that takes place during the establishment of a pregnancy.”

Read more: Abnormal timing in the womb may cause miscarriage