Implantation

Implantation occurs on the 6th day after fertilisation. The zonal pellucida is a protective covering around the blastocyst that prevents it's implantation into the uterine tube. It disappears on the 5th day after fertilisation. The endometrium of the uterus has been converted into decidua by now. The trophoblast has proteolytic action which, promoted by the uterine mucosa, causes implantation of the blastocyst. By the 12th day (after fertilisation) implantation is complete and by the 13th day the surface defect in the uterus has healed. Increased blood flow into the lacunar spaces may cause some bleeding(12th day after ovulation, ie, fertilisation the blastocyst is implanted+14th day of menstrual cycle is ovulation=26~28) which may be mistaken for normal menstrual bleeding which usually occurs around the 28th day of the cycle. The trophoblast later forms placental villi.