What happens in week 1?

After fertilisation, the genes carried by the egg and sperm combine, creating the complete set of instructions needed to make a new person. The fertilised egg is called a zygote. About 30 hours later, this single cell starts to divide - into two, four, then eight cells, and so on. Meanwhile it travels down the fallopian tube and six days later, the resulting clump of cells burrows into the lining of the womb. Only about 75% of all fertilised eggs get as far as this stage.

Electron micrograph of a four-day-old human embryo.