How Embryos hatch?
Prior to an embryo there exists an egg or an oocyte, and a sperm. The oocyte consists of a protein shell and fuses with the sperm as a first step towards becoming an embryo. The fusion of the protein shell and the sperm is the start of the fertilization process. Following the fertilization the shell hardens and prevents other sperm cells from entering the now-fertilized zygote.
This is also important to ensure that the embryo does not get implanted prematurely in the fallopian tubes and reaches the intended destination in the uterus walls. As the zygote travels through the fallopian tube into the uterus, it develops into a blastocyst and the outer shell begins to expand, thin out and eventually deteriorate.
This process takes around four days, wherein the exterior shell cracks open and the blastocyst/embryo emerges out of the protein shell. This is the stage that is known as embryo hatching. Following this, the blastocyst gets implanted into the endometrium and can detect a pregnancy.