Last Updated: April 10, 2019

Definition - What does Implantation mean?

Implantation is the process where a fertilized egg becomes embedded within the uterine lining. It is the first stage of pregnancy, in which a life-sustaining connection is developed between the fertilized egg and the nutrients and oxygen provided by the uterine wall. Optimal implantation occurs at the top of the uterus.

Implantation may also be known as embryo Implantation or blastocyst implantation.

FertilitySmarts explains Implantation

For implantation to occur, the uterus needs to be highly receptive and the membranes of endometrial cells change to encourage the adhesion of a blastocyst. New nutrient-rich endometrial cells, designed to nourish a blastocyst, are produced. This tissue will be shed in the next menstruation if implantation does not occur.

If implantation does occur, the uterus will continue to sustain these nutrient-rich cells instead of shedding them and menstruation typically will not occur. This receptivity occurs roughly 6 days after ovulation and lasts around 2 to 4 days, or through days 20 to 24 of a menstrual cycle

Implantation starts around 6 days after fertilization. Most successful pregnancies implant 8 to 10 days after ovulation. The risk of miscarriage increases with later implantation.

The steps in blastocyst attachment are:

  • Adplantation - Occurring around day 6 or 7, the blastocyst makes initial contact with the endometrium. The blastocyst "rolls" across the surface until the inner cell mass aligns with the uterine surface.
  • Adhesion - Occurring around day 7 or 8, the cells on the outside of the blastocyst (called trophoblast cells) have special molecules that stick to the cells of the endometrium (called epithelial cells). This superficial implantation allows the embryo to receive nutrients directly from cells within the uterus.
  • Invasion - Once attached, around day 8 or 9, the trophoblast cells on the blastocyst release enzymes that liquefy and separate cells within the endometrium, eventually burrowing a tunnel inside the uterine wall. The trophoblast cells also provide nutrients to the inner cell mass. Around day 12 a coagulation plug forms around the surface of the uterine wall where the blastocyst entered, creating a snug environment.

Signs of successful implantation include nausea, changes to taste perception and food preferences, a slight rise in body temperature, a missed period, and breast tenderness. Common changes to food preferences include a sudden distaste for meat and fish and sudden cravings for salt. These changes are brought about by hormones released by the new embryo.

Home pregnancy tests can confirm a pregnancy by detecting the hCG hormone produced by the growing placenta as early as a week after implantation - usually about two weeks after the date of last ovulation, or four weeks after the date of last menstrual period.

Implantation outside of the uterus is called an ectopic pregnancy and is not viable.

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