Obstetric dating accuracy
If your womb (uterus) is very deep in your pelvis, or if you're overweight, you may be offered a vaginal scan, which can get a closer view of your baby.
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.
That will happen at your next routine scan at about 20 weeks However, some major problems can be seen at your dating scan, such as issues with the umbilical cord or with your baby's skull or abdominal wall. You may know when your LMP was, but not when you actually conceived. If you do know the conception date, this is counted as day 14, not day one. 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. How big your baby will be depends on when you have the scan: After 13 weeks, your baby can curl up and stretch out, so measuring the length becomes less accurate.