A Missed or Abnormal Period
Some women's periods are like clockwork -- they arrive every 28 days, rain or shine. If you happen to be one of these lucky ladies, you may find it easier to notice if your monthly miracle doesn't arrive, which can be one of the first predictors of pregnancy. However, many women during early pregnancy experience spotting or light vaginal bleeding that lasts a couple of days and may be confused with a period.
This occurs when the fertilized egg nestles into the wall of the uterus approximately one to two weeks after fertilization [source: Dyne et al.]. Still, a delayed or missed period isn't always a perfect predictor of pregnancy since stress, travel or health complications can also stall or stop the tick-tock of the menstrual clock.