Many people are surprised to learn that the fertilization of an egg by sperm happens in the tube, not the uterus. They’re also shocked to hear that after ovulation, the fertile window during which an egg can be fertilized is only twelve to twenty-four hours because the egg is viable for only that short time. If a sperm cell successfully fertilizes an egg, the developing embryo travels down the rest of the tube and implants in the uterus about five to six days after fertilization. An embryo exists for almost a week before it implants in the uterus or produces any detectable pregnancy hormones.
