@ElizaBennett - I never experienced implantation bleeding myself, but I think it's one reason why some women don't find out they're pregnant until they're kind of far along. If they have had light, irregular periods before, they could easily mistake implantation bleeding for a light period. But for most women, a period will mean they are definitely not pregnant, especially if there is bright red bleeding (as opposed to pink or brown spotting).
It's so weird to me that doctors count pregnancy from the last menstrual period, so that by the time you conceive, you are already about two weeks pregnant! But keeping track of ovulation can lead to a much more accurate due date. Most women will deliver their babies about 266 days after ovulation, give or take a week on either side.
But ovulation doesn't always occur on day 14 of the menstrual cycle! It might be early or very late. If a woman ovulates late, say on day 25, her doctor might think her baby is a week "late" when really baby is not done growing yet! (Don't know when you conceived? Ask for an early ultrasound to verify the age of your baby. The earlier it's done, the more accurate.)