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What happens during the first two weeks after fertilization?

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Fetal growth — human development Growth of the human fetus from the fourth month to the ninth month of pregnancy.

The first two weeks after fertilization, known as the pre-embryonic stage, are characterized by rapid cell division and initial differentiation.1

Cleavage and Blastulation

Following fertilization in the fallopian tubes, the zygote undergoes a series of mitotic divisions called cleavage, creating smaller cells known as blastomeres. By the fourth day, a cluster of about 12 blastomeres, called a morula, enters the uterus. As division continues, fluid accumulates to form a hollow structure called a blastocyst. This structure consists of an inner cell mass, which will develop into the embryo, and an outer layer called the trophoblast, which contributes to the formation of the placenta.234

Implantation

After remaining free in the uterine cavity for about a day, the blastocyst attaches to the endometrium (uterine lining) around the fifth or sixth day. The trophoblast cells then actively invade the uterine tissue, allowing the blastocyst to burrow into the lining. By the 11th day after fertilization, the conceptus is typically completely embedded within the endometrium, marking the successful completion of implantation.34