||I was taught that it is important to ask a qualified Posek re:seeing blood late in pregnancy.
There are many reasons why a woman might think she is seeing "blood" in late preg. that might not render her a nidda.
It is always important to call your caregiver if you see blood in late pregnancy, but also unless you know for sure that labour is definitely well established, you should also ask a Rav or you might walk around for the last weeks of pregnancy thinking you are niddah when you might not be.
There are enough things that women deal with in late pregnancy(exhaustion,backache,pressure,etc, etc) and believing you are niddah for weeks (the postpartum period is long enough) when you might not be should not be one of them. Please ask a shaila!!