A pregnancy results from the completion of ovulation, fertilization, and implantation.
The process begins when an oocyte is released from the ovary and travels into the uterine tube.
If intercourse is performed, sperm travel from the vagina into the uterus, and through the uterus into the uterine tube. One sperm fertilizes the oocyte. The newly formed zygote divides by mitosis.
When this zygote reaches the stage where it is made up of four cells, it is called an embryo.
The embryo is called a morula when it develops into a solid ball of 16 cells.
The cells keep dividing, and a fluid-filled cavity develops in the embryo. It is now called a blastocyst.
The blastocyst implants in the lining of the uterus, called the endometrium. Attachment of the blastocyst causes nearby endometrial cells to grow quickly. In five days, the blastocyst is fully embedded and implantation is complete.