Researchers at the University of Cambridge have made a discovery that could explain why some pregnancies fail.
The researchers found a group of cells outside the embryo which triggers the first steps of human development.
Known as the hypoblast the event occurs in the second week of gestation. It is the first step in the formation of the overall body pattern in humans.
It is thought the findings could help in the understanding how human life begins and open the door to knowing more about why early pregnancy loss occurs.
Currently not much is known about embryo development once it implants in the uterus, due to ethical restrictions on use of human embryos in research.
In 2016, the team at Cambridge worked out a way to culture human embryos outside of the body, allowing them to be studied for two weeks.
They found that the hypoblast sends a message to the embryo that kick-starts the development of the head-to-tail body axis, where one end becomes committed to develop into the "head" end, and the other the "tail".