The process of ovulation, fertilization, and implantation must all occur before a pregnancy can begin. First, a mature oocyte emerges from the ovary (ovulation), is taken up by the fallopian tube, and meets and binds to sperm (fertilization.) The fertilized oocyte is carried through the fallopian tubes toward the uterus as the first cell divisions occur. It adheres to the endometrium and burrows. This is implantation. Compared to ovulation and fertilization, there is much more about implantation that is poorly understood by modern fertility science. The time when this can happen is limited to 3-4 days in one menstrual cycle, and during this period both the fertilized oocyte and the endometrium must be in good condition.