Is it a 50/50?

I always thought that when a woman had a baby, there was a 50% chance the baby would be a boy and a 50% chance the baby would be a girl... However, people are telling me that the more you have of one sex, the more likely you are to have another one of that sex... and each time you have a child of the same sex, the odds change making it no longer a 50/50. This makes no sense to me. Why wouldn't it be a 50/50 every time? We'll be happy either way and are excited to be surprised on the big day.... but if you can explain this to me, I'd be very interested in understanding the logistics!