You were already pregnant on the 17th. If your estimated due date from the doctor is accurate, you would have become pregnant from sex you had sometime between March 29 - April 3rd. You "conceive" on the day you ovulate, but you can have sex up to five days before that and the sperm can still potentially fertilize the egg you release.
If you use a "due date calculator" online it usually goes by first day of last period. If you don't know it, you could put in March 20th and that should give you a due date of December 25th
at 10:38 PM on Jun. 23, 2013