How many weeks is a full term pregnancy?

It's forty weeks from the last missed periods. Forty weeks is 240 days. That comes to ten lunar months. I have always been wondering about these 'ten' figure. This figure is known to the nature from ages. Incidentally the time period between two periods is one lunar month. This can be easily explained. It is the biological clock that is related to the natural event of rotation of moon around the earth.