Imagine a tiny clump of cells that looks just like a human embryo. Scientists have created this amazing model using stem cells—no eggs, sperm, or womb involved. This lab-made embryo has a yolk sac and a proto-placenta, which are features seen in real human embryos about 14 days after fertilization. Remarkably, it even produces hormones that can turn a pregnancy test positive!

Jacob Hanna, a professor at the Weizmann Institute of Science in Israel, was thrilled when two students rushed in...