It is estimated that the majority of pregnancies that fail do so within the first seven days after fertilization, before the embryo implants into the uterus. In this time period, a complicated cascade of events occurs with precise timing. One particularly important process is called polarization, when the individual cells that make up the embryo become asymmetrical. Polarization occurs at 2.5 days for mouse embryos and 3.5 days for human embryos.