ok this makes since!!! U conceive at 2 weeks pregnant. U cant get a + hpt until 4 + weeks pregnant. so If u were 9 weeks on sept 12th then u more then likely conceived around the 25th of July matching up perfectly with lmp ( last men period. U took a test before u even missed your next period. u would have only been about 3 weeks. that is about the time of implantation meaning there is no hcg or very little.
it even works with yyour number of weeks u are.
http://www.babycenter.com/pregnancy-due-date-calculator?sub=true&lmp=true&month=6&day=10&year=2011
gl
if you know the weeks you should be able to count back. obviously you conceived in july, some women don't have high enough hcg levels to get a positive result at the first missed period. sometimes it takes longer for them to get their positive. but if you have the doctors say that you are 9 weeks already conception date is irrelevant, unless you are trying to figure out who the daddy is. as long as you have a due date i wouldn't worry too much about the day it happened (unless you have two partners) just count backwards. (ovulation is usually 14 days before your next expected period. if you are a 28 day cycle then its day 14. doctors go by the first day of your last period as your conception day as every woman is different. and due dates really are just guess days. but your second child will likely have the same gestational period as the first.