When does a group of cells become a human? I'm not sure, but most countries agree that scientists are allowed to grow human embryos in a lab for a maximum of 14 days.
Nobody had ever come up against this two-week rule because keeping an embryo alive outside a womb is notoriously difficult. But for the first time, a team of scientists have succeeded in implanting a human embryo into an artificial womb. They studied it for 13 days before stopping the research ahead of the legal limit.
This has revealed new insights into early human development and brought up some serious ethical questions.