Go by your last period. Normally 11-16 days after the first day. The dr changed it ..based on the size of your baby right? Could just mean you have a healthy quick growing baby :)
Y0u usually ovulate 14 days before your missed period or midway thru your cycle. Keep in mind every baby grows at diffrent rates and the further along you are the harder it is to tell when you concieved.
My last period date was Dec 5th I found out I was preg the first week of January . This whole pregnancy calendar has me confused ..