At which stage of development does the embryo implant in the uterus?

Blastocyst stage

Explanation:

A baby undergoes different stages of development before they become fully matured. It all begins with the process of fertilization which involves a sperm cell coming in contact with an egg.

After a successful fertilization, the fertilized egg travels down to the uterus where it undergoes series of cell division till it becomes a stack of arranged cells known as the blastocyst.

During the blastocyst stage, the embryo fastens itself to the edge of the uterus, this process is known as implantation.

Hence, the embryo implants in the uterus during the blastocyst stage.