"Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age" .
Years ago missing a twin even with ultrasound was a greater possibility early on in a pregnancy.
Babies with abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this clear space to be larger than average. The NT scan must be done when you're between 11 and 14 weeks pregnant, because this is when the base of your baby's neck is still transparent.
(The last day you can have it is the day you turn 13 weeks and 6 days pregnant.) It's usually offered along with a blood test in first-trimester combined screening options.(Some practitioners also look for the presence of the fetal nasal bone in the first trimester.
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.
This means that accurate dating reduces your chance of having your labour induced because your baby is thought to be overdue.