Got this off a website on infertility !!!!
After fertilisation
* The fertilised egg moves down the fallopian tube to the womb. This takes several days.
* It starts growing and embeds in the lining, called the endometrium.
* Now it's called an embryo. It forms the placenta to get food from its mother. It's only at this stage that pregnancy has started.
* The placenta makes a special hormone to make the lining of the womb thicken. The hormone also prevents the woman having her period.
So - several days - I guess it depends how far down the fallopian tube it is when it gets fertilised.