Scientists Grow Mouse Embryos in a Mechanical Womb Biologists have long held that a fetus needs a living uterus to develop. Maybe not anymore. Video Mouse embryos, their beating hearts visible, grown outside the womb after four days.CreditCredit...By A. Aguilera-Castrejon Et Al., Nature 2021 March 17, 2021 The mouse embryos looked perfectly normal. All their organs were developing as expected, along with their limbs and circulatory and nervous systems. Their tiny hearts were beating at a normal 170 beats per minute. But these embryos were not growing in a mother mouse. They were developed inside an artificial uterus, the first time such a feat has been accomplished, scientists reported on Wednesday.