If you had had the ultrasound earlier, your due date could be used to reliably pinpoint conception as around April 4-6. But even with the ultrasound being in your 18th week, your dates won't be off by more than two weeks. This rules out Mr. March 18.
I'm actually having identical twins. The edd was December 28 and December 26 but that was at the 18 week ultrasound. Twin a measured 18 w 4 and twin b 18 w 6.
Jeanie, don't feel hopeless. You are one of the luckiest women around, because you are going to have a child. Talk to any of the ladies on the infertility forum if you can't believe this right now. Anyone coping with infertility or secondary infertility would be willing to sell their house to be pregnant. You have won that lottery! You are so lucky; you are going to have a child to love.
You have given a lot of dates, but haven't mentioned an estimated due date given to you by the doctor doing the ultrasound. (I am not entirely convinced by fundal height measurements.) What e.d.d. has been given to you and at what point and by what method?
I should add that my periods are very irregular but I had one sometime the week of march 24-27. It wasn't very long but that is common with me.