I had sex on sept 27, which was the first day of my period. today is october 31, and still no period. could i be prego.

Possible, but...: Pregnancy is the most common cause of a missed period. Other causes of a late or missed period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (such as birth control pills), hormonal disorders, etc… A home urine pregnancy test is very accurate if it is done when a woman is a week late for her regular period.