Hi Irena
The baby would mormally have a heart beat between 6 and 7 weeks.
but do not worry because the U/S is only accurate to within 1 week. This means that when they did the U/S scan to them the baby could have looked to be anything from 4 weeks 5 days to 6 weeks 5 days. If your are sure of your dates then I would not expect to be able to see a heart beat 5 weks 5 days

Hope this sets your mind at rest.