Ez - in response to you comment... is due dates changing.

They go off the ultrasound if you had one.

If you go off your period you need to remember you ovulate 2 weeks BEFORE you period NOT 2 weeks after your period... if that makes sense.

So even if you had your period for example 1st jan... you may not have ov'ed until 24th? so that is why a week is lost.....

sorry prob confused you more