Implantation in Human

The process of the blastocyst embedding itself into the endometrial lining of the uterus about 5–7 days after ovulation is called implantation.

or

The process of fixing of the blastocyst to the wall of the uterus/endometrium is termed implantation.

From zygote to blastocyst: passage of growing embryo through the fallopian tube

After fertilisation, the zygote starts dividing mitotically as it moves through the isthmus of the oviduct towards the uterus - this is called cleavage. The daughter cells are called blastomeres. Cells form in the sequence 2 → 4 → 8 → 16. The overall embryo size does not increase — only cell number increases. 

The morula continues to divide and transforms into a blastocyst as it moves further into the uterus - this process is known as blastulation. The blastocyst is a fluid-filled hollow ball of approximately ~100 cells or ~64 cells. The fluid-filled cavity inside is called the blastocoel.

Implantation of the fertilised ovum outside the uterus is called ectopic pregnancy. It is a life-threatening medical emergency requiring immediate intervention.

• The zygote divides by mitosis (cleavage) while moving towards the uterus.
• It forms blastomeres, and the 8–16 cell stage is called morula.
• Morula develops into a blastocyst with trophoblast (outer layer) and inner cell mass.
• Trophoblast attaches to the uterine lining (endometrium), and the inner cell mass forms the embryo.
• Implantation is the embedding of the blastocyst into the endometrium, leading to pregnancy.