Define "Lie"

I have been having a running discussion with one of my co workers as to what defines telling a lie. I say that omitting the truth, when directly asked a question, is a lie. She says it isn't. This all goes back to when I hired a girl a few months ago. I asked her if she had any children. This is important, as the position is for a one person reception job. If she is not able to come in, there would be no one to sit at the front desk. I know that people do get sick, but my previous experience is that women with children have a higher absence rate. Had this not been a one woman office at night, I would not have cared. When I asked the question, the girl looked me straight in the eyes and said that she had no children. I was quite annoyed, when three weeks later, I found out, through the grapevine, that she was 7 months pregnant. She did not have a big belly, so no one knew, until the day her stomach popped. Since she was already hired, I cannot fire her. She has now been out of work for 5 weeks. The baby is one week late and if she has a C-Section, it will be another month at least until she is back. This is now a politically correct world, so I cannot fire her. I have random people filling in for her on a daily basis. This is not good. That is where the debate of the word "lie" comes in. I was lied to and now my office is suffering.