There are several early signs of pregnancy, but it is easy to confuse them with the symptoms of PMS.
For instance, a woman in the pre-menstrual period may experience bloating, fatigue and irritability. These signs are amongst the first signs of pregnancy too. This could explain why most women remain unaware that they are pregnant till more obvious symptoms appear.
Despite the woman’s lack of awareness about the pregnancy, the embryo already begins to develop inside the uterus. As it grows, the body gives out signals about the happy event – but which may be confused for menstrual period symptoms. The first (most obvious) sign of pregnancy is a missed period. This is easy to spot when the woman is totally clued in to her menstrual cycle and is aware of the expected period date. If she has been sexually active during this time and misses her period, it will certainly ring an alarm bell in her mind.
However, once the embryo positions itself in the uterus, there may be a brief period of spotting or light bleeding through the vagina. The woman may mistake this for a delayed period, and will certainly notice that the period is very light. This bleeding occurs as the embryo implants itself in the uterus, and it tapers off after a few days.
This is followed by tender and sore breasts. Your breasts will feel fuller – this happens because the body retains more water after the pregnancy occurs. Your midriff will also feel bloated due to water retention.
Some women feel fatigued, and need to nap for short periods of time. This is among the first signs of pregnancy as well, and it is normally accompanied by increased thirst and urination.
 Stewart, Rebecca Felsenthal, ‘8 early signs of pregnancy’, http://www.webmd.com/baby/features/8-early-signs-of-pregnancy