Um. I was confused by this when my wife was pregnant. The due dates -- from the gynae, phone apps, etc -- are worked out as nine months from when she last menstruated, which is two weeks before she ovulated and therefore at least two weeks before she could ever have been pregnant. There seems to be no way of knowing precisely when the fertilisation took place, so they do this instead. It's weird, but I think it's fairly standard practice. At least in my experience... *shrug*
Seems like standard practice, see post#15.