I had an early scan a week and a half ago as a had some heavy bleeding. The scan showed a healthy fetus with a strong heartbeat. The nurse said the fetus was very small (had to do a trans vaginal scan to find it) so only looked around 4 and a half weeks (it was 1.8mm) however she said it was probably more like 5 weeks because they found a heartbeat so strong. When I work back my dates myself, my last period was 26th jan making me 7 weeks?? Now I'm worried I am 7 weeks and the baby isn't developing properly and am wondering what the chances of miscarriage are after a heartbeat is found? Thank you x

The difference in dates can come from dating from your last period and dating from when conception likely occurred. You likely became pregnant about midway through a cycle, or about two weeks after your period. This is where the 5 weeks comes from... fertilization age. Dating from your period is often used and this is gestational age. This is why you have a four week window at the end of pregnancy for what would be considered a normal time to deliver.
While miscarriage can occur, the odds go up for a successful pregnancy once the heartbeat is found. Wishing you a smooth pregnancy!

Thank you for taking the time to answer, it was very helpful and everything is so far so good!