My first one was a girl, and I was very tired, had sore breasts, and found myself feeling nauseous in the mornings and when smelling certain foods.
My second was a boy, and when I missed my period, I did a pregnancy test that came back negative. I figured my cycle was messed up, so I continued taking the allergy medicine I was on. A week later, still no period and still no symptoms. I took another test that also came back negative. Shortly after that was my annual, and as soon as I told my OB/GYN what was happening, he knew I was pregnant. He said the lack of signs was normal, as were the negative tests. Of course, I had to stop taking my allergy meds, but he said not to worry about what I had been taking since becoming pregnant.
That second pregnancy didn't get difficult until my third trimester, when all the normal discomfort stuff sets in.