Most women don't start to feel symptoms until after their missed period (at which point they are considered 4 weeks pregnant). Morning sickness usually comes on at 6-8 weeks pregnant (2-4 weeks after the missed period). One of the earliest symptoms for most women is extremely sore & tender breasts, which they may notice shortly after missing their period, but for me, my breasts didn't start gettting sore until I was around 10 weeks pregnant.
I'm almost 12 weeks pregnant. For me, my only obvious early symptom was a late period (my cycles were extremely regular, though and this is not the case for a lot of females especially teenagers). Other factors like stress, diet, exercise can cause late periods too.