Implantation occurs after 7-8 days of fertilisation. The fertilised ovum starts dividing and then from the morula (8-16 cells) the embryo transforms to blastocyst (100- cell stage). The outer layer of blastocyst is known as the trophoblast, it is highly sticky and is covered by a layer of zona pellucida, which prevents implantation of blastocyst in the fallopian tube. In 3-4 days, blastocyst is transported to the uterus. Their zona pellucida disappears and the trophoblast layer attaches with the endometrium of the uterus and the embryo is implanted.
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