Crown-rump length (CRL) and first-trimester ultrasound dating

Crown-rump length (CRL) is the measurement from the top of an embryo's head to the bottom of its torso, taken by ultrasound. In the first trimester it is the single most reliable way to date a pregnancy — more reliable than the last menstrual period — which is why the “dating scan” is offered early in pregnancy.

Why CRL is so accurate early on

In the first weeks, embryos grow at a remarkably consistent rate from one pregnancy to the next. That consistency means a single length measurement maps cleanly onto a gestational age. As pregnancy advances, genetics and environment cause growth to diverge, so a measurement taken later tells you less about dating and more about size. That's the core reason an early scan dates better than a late one.

From a measurement to a due date

The sonographer measures the CRL in millimetres and software converts it to a gestational age using a published growth formula. From that gestational age, the due date is just the date the pregnancy reaches 40 weeks. If you already have the “X weeks Y days” figure from a scan report, you can reproduce the dating in our ultrasound mode: enter the scan date and the gestational age measured that day, and it returns the due date and the current gestational age.

CRL is typically used up to around 13–14 weeks (roughly an 84 mm measurement). Beyond that, dating switches to other biometric measurements such as head circumference and femur length, which become the better yardsticks in the second trimester.

How accurate is it?

A first-trimester CRL measurement dates a pregnancy to within about five to seven days in most cases — tighter than menstrual dating, which depends on remembering the LMP and on a textbook cycle. Accuracy is best earliest and gradually loosens as pregnancy progresses.

When the scan overrides the last period

Guidelines from ACOG give a practical rule: if the ultrasound date differs from the last-period date by more than a set margin, re-date the pregnancy using the scan. That margin is small early on and widens later, reflecting how dating precision decreases with gestational age. In broad terms, a difference of more than about five days before nine weeks, or more than about seven days from nine to roughly sixteen weeks, is enough to switch to the scan date. Your clinician applies the exact thresholds; the takeaway is that a good early scan usually wins.

Putting it together

If you have a dating scan, use it. If you only have your last period, menstrual dating via Naegele's rule is a reasonable estimate to start with. Either way, the due date is an estimate — see how accurate is your due date — and dating decisions belong with your provider.