That said, it doesn't matter what week the sonogram is taken to determine its accuracy, because it is based on the CRL measurement of the fetus.No matter what the length is (because it was done on different weeks of gestation) is will directly relate to the week of gestation and ultimately determine the EDD (estimated date of delivery).By taking the due date you have been given, count back 40 weeks.The date you get is only a rough estimate of the date of conception. The day you recall having sex should be within that week, either before or after the date you counted back to.Keep in mind, however, when trying to figure your conception date that a full term pregnancy is considered to be 38-42 weeks, and that the accuracy of the sonogram being plus or minus 3-5 days.
You may have had sex anywhere from May 3 to May 10, or May 10 to May 17 and conceived up to 72 hours after you had sex in between May 3 to May 17about.
Last bleeding, to determine if it was a normal menstrual period or lighter bleeding which may have been implantation bleeding.
Plus, menstrual history in general, length of cycles, etc.
Also, an egg doesn't become fertilized and implanted immediately after ejaculation.
It can take up to 72 hours post-sex before that happens.
Here are some ideas to help you figure an accurate due date.