Sperm and ova have only 23 chromosomes, so that when they meet, the single fertilized cell that results has 46. The fertilized cell is called a zygote and as it travels down the fallopian tube towards the uterus it divides repeatedly until it becomes a hollow clump of about 100 cells called a blastocyst.
A week after fertilization the blastocyst secretes a hormone which helps it to burrow its way into the endometrium, the lining of the uterus. Implantation is usually in the upper third of the uterus. The pregnancy is now established and the placenta starts to form.