It is during the second trimester that you will begin to “show”, that is, your tummy will begin to bulge out. The exact timing is different for each pregnancy.
Generally speaking, first babies will show later in the second trimester then subsequent pregnancies. I think this is because of decreased elasticity of the tissues as we age and as the tissues have been stretched out before. Think of a balloon. A balloon gets easier to blow up each time it is expanded.
There is an uncomfortable time period for many women when the weight is coming on but they don’t look obviously pregnant yet. Women tend not to like this because they feel like people just think they are getting fat. Your tummy will pop out, don’t worry. For many people it seems to happen overnight, one day they hardly look pregnant, the next day they are obviously showing.
Try to accept your body in all of its changes. You are supposed to gain weight (15 – 35 pounds depending on your weight prior to pregnancy). It seems that women think they are too big or not big enough. Try to embrace the pregnancy changes and be happy that your little baby is growing and developing.