Sperm might last a long time in a person's system, but not ten days, as I understand it.
Yes, it the last ultrasound, which seems to predate the date of my last menstrual period that has alarmed me most... was unsure if this just meant the baby had a growth spurt and so was bigger at this point than usual...
My concern is that any sperm from 24th March could have lived through my period in my fallopian tubes past 4th April (as i defniately know that i still hadn't ovulated at this point because of ovarian U/S).
The internet can be a scary thing as it suggests that some can live this long, that conception prior to a period is possible and that ultrasounds can be out by two weeks (even though they have all been consistent with an EDD of 29 December) so i have worried myself sick and made my self paranoid which isn't good when you're pregnant. Thanks for your help and any more you can offer, it is much appreciated indeed as even the sonogrpahers haven't really been much help when have asked for explanations.
Use the first ultrasound. Babies grow at different rates, so for the purposes of dating, only the ultrasounds under 9 weeks are accurate enough. Is the problem that you are worried about the last ultrasound? If so, don't worry, it just means the baby is growing.
Your 8th of May ultrasound came out exactly at the first day of your last period, which is how it should come out, since pregnancy is dated the medical way, i.e., by the first day of the last period. (When an ultrasonographer says 6 weeks 2 days, she is not saying that is when you conceived. Se is saying that your embryo is the size of an embryo of a woman whose first day of her last period was 6 weeks 2 days ago. This would mean you conceived more like 4 weeks 2 days prior to the ultrasound.)
Even if the 15 weeks 3 days were entirely accurate (removing the variation in rate of growth of the embryo), it would only mean that you conceived about 13 weeks 3 days ago. sometime in early April.
There is really no chance you have gotten pregnant and then have your period!