Caltech researchers have developed a powerful new method for creating embryo-like structures from stem cells that could transform how we study fertility. Using stem cells rather than a traditional fertilized egg, the team has built mouse embryo models called iG4-blastoids that closely mimic natural blastocysts, the stage of development when an embryo implants into the uterus. This implantation stage is when the majority of pregnancies fail, including those resulting from in vitro fertilization,