If you do know the conception date, this is counted as day 14, not day one.
The dating scan isn't designed to look for more subtle abnormalities, though. 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. As with all screening tests, there can be false-positive and false-negative results. Implementation of maternal blood cell-free DNA testing in early screening for aneuploidies. Finding out about having twins early in pregnancy also gives you more time to prepare for the birth and for your doctor and midwife to plan your care. Nearly all scans after 10 weeks can be done through your tummy, and most units ask you to arrive with a full bladder. The sonographer will put some gel on your tummy and move a small hand-held device (transducer) over your skin to get views of your baby. The dating scan will also show if you're expecting twins, triplets, or more. It's helpful to know about twins early on, and it's easier to see whether or not they share a placenta during 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. Therefore, from 13 weeks, or if your baby is longer than 84mm, the circumference of the head becomes the best way to measure your baby. Evaluation of gestation emedicine.[Accessed July 2017] Newson L. The scan will usually show that your pregnancy is less far along than suggested by your LMP. This means that accurate dating reduces your chance of having your labour induced because your baby is thought to be overdue.