A week or two after being conceived, the human embryo is just a tiny ball of cells. But something incredibly important happens around this time called implantation. This is when the embryo attaches to the wall of the mother’s uterus and embeds itself into the tissue. The cells on the outside of the ball then start to form the placenta, which is the food and waste transport system 5 linking the mother with her developing offspring. After implantation, the mother deploys large numbers of a special kind of white blood cell to the placenta.

These are called “Natural Killer”, or “NK” cells, which are normally responsible for killing virus-infected cells in the body. In pregnancy, though, they have a very different role. Scientists have known for some time that these cells are important for the healthy development of 10 the placenta and its blood vessels, but the full picture of their contribution wasn’t appreciated. Now researchers at the University of Science and Technology of China took uterine NK cells from pregnant women and studied the pattern of genes that active in the cells. A subset of the cells was producing growth-promoting factors, known to trigger the growth of bone, cartilage and blood vessels in an embryo.

The researchers then compared healthy pregnancies with those that ended in 15 miscarriage. Pregnancies ending in miscarriages, they found, often had fewer of these growthpromoting NK cells, suggesting that NK cells contribute to embryo development through the expression of these growth factors. These findings could have important clinical implications. If a woman who wants to have a child doesn’t have sufficient growth-promoting NK cells in her uterus, it’s possible that her 20 pregnancy will end in miscarriage. But if these cells can be grown in the lab, the team speculate, they could be added to her uterus to help the embryo develop normally.