Fertilisation and early development
Following fertilisation the zygote starts to divide, around once every 24 hours, becoming 2 cells, then 4 cells and 16 cells over the next 3 days. As this ball of cells divides it continues to travel towards the uterus (womb).
Around 5 to 8 days after ovulation, the ball of cells opens out to form a fluid filled centre. It is now known as a blastocyst. The blastocyst comes to rest on the uterine wall, or endometrium, where it becomes buried or implanted. Cells at one end of the blastocyst begin to undergo specialisation to form three distinctive layers of germ cells. These will develop into the embryo.
Be aware
Following conception, the embryo begins to secrete a hormone called human chorionic gonadotropin or HCG. This prevents the menstrual cycle continuing and maintains the uterine wall. Pregnancy tests look for the presence of HCG in urine or blood. They are very sensitive and may register the hormone before the egg is fully implanted and pregnancy established.View the video for an overview of fertilisation and early development.