Maybe it works on an estimate based on the amount of hormone that should be in your body, possibly make a doctors appointment and ask if they can do a blood test to check the HCG levels for you. When I was pregnant with my first one, they thought he was ectopic, with all the pain and massive amounts of bleeding they often thought he was a miscarriage and did repeat pregnancy tests, and blood tests I had one after my 20 week scan that barely registered as being pregnant because my HCG levels were just so low. They could see from all the ultrasounds that he was fine (even if they did think he was a girl lol). Every single one of mine has been smaller on the scans than the dates say they should be, it's especially amusing when you can tell them the exact day you had sex on, they work out the widest possible conception window and they still can't get the dates to measure up to the babies. Hugs Crystal xx