Early Embryo Development-Missouri Center for Reproductive Medicine & Fertility

Text: Early Development of the Fertilized Egg...

After fertilization, the fertilized egg, now called a zygote, grows and divides until it contains 8 cells, about 3 days after fertilization. During this time, it moves through the Fallopian tube and enters the uterus about 4 days after fertilization as a compacted ball of cells called a morula.

The Morula then develops into a blastocyst about 5-6 days after fertilization. The blastocyst is a circular structure filled with fluid. The cells surrounding the outside of the blastocyst form the future placenta and a smaller area of cells on the inside form the future fetus. The blastocyst is the final stage of development before implantation.

Drawing of the female reproductive tract with the ovary, uterus and Fallopian tube labelled and the Day 1 fertilized egg, Day 2-cell ovary, 4-cell embryo, Day 4 morulla, and Day 5-6 blastocyst in the uterus

See actual pictures from our laboratory on our Preimplantation Embryo Culture page showing the developing preimplantation embryo with the important structures labeled.