january 20th if you were 18w3d on may 11th. (i subtracted 16 weeks 3 days from may 11th to arrive at this answer)

I ALSO GOT VERY CONFUSED WITH HOW AN ULTRASOUND DATED YOU AND WHEN ASKED MY MIDWIFE SHE EXPLAINED THAT THE ULTRASOUND DATES FROM COCEPTION NOT INCLUDING THE TWO WEEKS YOUR NOT PREGNANT SO IF THEY SAID YOU ARE 18 WEEKS AND 6 DAYS YOU COUNT BACK 18 WEEKS AND 6 DAY ALLOWING 3 DAYS BOTH SIDES HOPE THAT MAKES SENSE AND HELPED X

all of my ultrasounds have fun based off of last period, not conception date. hers cant run off of conception date if she is asking what her conception date is... because she doesnt know her date of conception! they would have imputed the first day of her last period in order to get gestational age of the fetus and went off of measurements of the baby during the ultrasound. a woman is actually only pregnant for 38 weeks, but we say 40 because the 2 weeks prior to conception are added to the beginning. you can look it up on any pregnancy site.