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A couple is told fertilization happens in the fallopian tube, not the uterus. After ovulation the egg waits in the ampulla of the fallopian tube; sperm swim up and one penetrates — 23 chromosomes join 23, restoring 46 in the zygote. The zygote divides while traveling to the uterus, arriving as a blastocyst around day 5, then implants in the endometrium on day 6–7. The implanting embryo secretes hCG — the hormone pregnancy tests detect. IVF simply performs fertilization in a dish and places the embryo in the uterus.