There is a normal, but often not documented, phenomenon when a couple is TTC (Trying To Conceive) called implantation bleeding. It often goes undocumented because many women do not know about it, don't observe it, or mistake it for something else. Implantation bleeding is a light spotting of blood roughly a week to 10 days after ovulation. If the egg is fertilized, it implants into the uterus and could cause a bit of bleeding. Often this is mistaken for the start of a woman's period.
The difference, with woman with a normal cycle, is the time. Implantation bleeding usually starts 7-10 days after ovulation whereas early menstrual spotting would occur on the 14th day or soon thereafter. Also, implantation bleeding is just a slight pinkish or brownish discharge instead of heavier bleeding. Implantation bleeding is thought to occur in 20-30% of pregnancies and is considered common.