It shouldn't be uncomfortable, and you don't need a full bladder Your sonographer will give you a report of the scan, which will tell you exactly how many weeks pregnant you are. Keep it with your maternity notes so your midwife and anyone else involved in your care can see it. This measurement, known as the crown rump length (CRL), is very accurate in the first trimester. The dating scan will also show if you're expecting twins, triplets, or more.
The scan can check that your baby's heart is beating nicely and that he's developing normally. A blood test at what you thought was 13 weeks wouldn't be helpful if you were actually 16 weeks pregnant. It may lead to you being given a high chance for Down's syndrome, when your baby is actually developing normally (false-positive result). Addition of non-invasive test to improve screening for pregnant women. If you do know the conception date, this is counted as day 14, not day one. If your cycle is irregular or you've recently been on the pill, working out the date from your LMP probably won't work. If the scan date differs from your period dates, the scan date will be used.
It may show a very minor problem, or something that may get better on its own. onlinelibrary.[Accessed July 2017] Loughna P, Chitty L, Evans T et al. Fetal size and dating: charts recommended for clinical obstetric practice.