ask: which is the correct expected date of delivary?according to my last menstrual period or by scanning?which is correct?

Hi Anon

Thanks for your question!

Both ways are used and both usually give the same result. However, an ultrasound can sometimes show a few days in difference. Sometimes it's accurate, sometimes it's wrong due to the baby's size or estimated weight.

Most women (and doctors and midwives) use this method to calculate the due date: http://www.babycenter.com/pregnancy-due-date-calculator

Bear in mind there is no real due date that can be focused on- women are "due" generally from weeks 38 to 40, with a window from weeks 37 to 42.

I hope this helps and thanks for writing!
~Susan