It's entirely up to you whether or not you have a scan. You could have a dating scan without NT screening, or even change your mind on the day of your scan. However, conception doesn't actually occur until around 14 days after your LMP, or later if your cycle is longer than 28 days. Multiple pregnancy: antenatal care for twin and triplet pregnancies CG129. You may know when your LMP was, but not when you actually conceived. Ultrasound scans are considered to be safe for you and your baby when performed by trained professionals. However, sometimes, having a scan can cause unnecessary worry. Most units offer a screening test for Down's syndrome and other conditions at the same time as the dating scan.
This means that accurate dating reduces your chance of having your labour induced because your baby is thought to be overdue.
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.
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.