2 weeks from the first day of your last period. (i.e. If you started on the 7th your conception date is the 21st)
Lol well, they'll just go by the baby's measurements then. They aren't always accurate, though because each baby developed differently.
Ugh that was forever ago! Lol I can't even try to calculate.
Usually based on your last menstrual cycle and how long your cycles are. You ovulate halfway between cycles.