Can an ultrasound calculate age of a fetus?

There is no methods known to man today that can show exact pregnancy date.

To find this pregnancy date we use a combination of statistics and measuring.

Ultra sound is a great tool for the measuring bit, but the statistics is only statistics. Statistics tells you something about how common something is. For the most it will be quite accurate. For some there will be a variation of up to a week one way or another. For the very few, this variation can be more than 2 weeks.

It is however not important (in most cases) to know the exact date of conception/pregnancy. The baby is ready when it is ready and so will you be in order to have the baby delivered.