For the first 12 hours after conception, the fertilized egg remains a single cell. After 30 hours or so, it divides from one cell into two. Some 15 hours later, the two cells divide to become four. And at the end of 3 days, the fertilized egg cell has become a berry-like structure made up of 16 cells. This structure is called a morula, which is Latin for mulberry.
During the first 8 or 9 days after conception, the cells that will eventually form the embryo continue to divide. At the same time, the hollow structure in which they have arranged themselves, called a blastocyst, is slowly carried toward the uterus by tiny hair-like structures in the fallopian tube, called cilia.
The blastocyst, though only the size of a pinhead, is actually composed of hundreds of cells. During the critically important process of implantation, the blastocyst must attach itself to the lining of the uterus or the pregnancy will not survive.
If we take a closer look at the uterus, you can see that the blastocyst actually buries itself in the lining of the uterus, where it will be able to get nourishment from the mother’s blood supply.
Review Date 9/7/2021
Updated by: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.