In the past it's been thought that an egg eroding into the uterine lining would cause bleeding at the time because of a burrowing effect. It's doubtful whether there's any bleeding when this happens, and if so, it's too small an amount to notice. The myth persists because there are bleeding episodes in which no cause is ever identified and in which the pregnancy goes on successfully to term. Such a mystery that starts off so menacingly but ends so well begs for an explanation that must include a natural process. Implantation makes sense under these criteria but can't be proven.
Periods During Pregnancy
Many women ask me about having regular periods during their pregnancies. They're concerned because Grandma or a cousin had periods every month during pregnancy, and could that happen during this pregnancy, making the last menstrual period (whichever one that was) screw up the due date? They swear that these periods during pregnancy are true – that they really happened. It's false, all the swearing notwithstanding. Shedding one's layer of menstrual tissue is not compatible with life. The closest thing we have to this is shedding of decidual tissue (as described above). When Grandma swears that it happened, it's certainly the polite thing to listen with an open mind – just be sure to slam it shut by thinking about what's really going on in pregnancy. The cycling of hormones stops because a pregnancy causes the hormone levels to remain high. This is necessary for pregnancy to continue. There are no falls in the hormones, which is what causes a period, except right before labor. Most likely, Grandma experienced a subchorionic hemorrhage (as described above), bleeding intermittently, misinterpreted as cyclic. That would explain her first trimester "periods." Just being Grandma might explain the rest of her pregnancy's periods.
Although the above instances describe the causes of bleeding that do not indicate miscarriage, miscarriage should still be ruled out if you have any bleeding at all. And when one considers that the cramping of a threatened miscarriage can feel exactly like the growing pains of a normal uterus, it is fortunate that there are other tools to give you peace of mind.