The First Trimester Dating Ultrasound: What to Expect and How It Works
Everything you need to know about the 11-13 week dating scan, CRL measurements, and confirming your official due date.
Dr. Sofia Moretti, Ultrasonographer
Medical & Editorial Contributor
The first-trimester dating ultrasound—often scheduled between 11 weeks and 13 weeks + 6 days—is one of the most exciting and medically significant milestones of pregnancy.
Not only does this scan offer the first clear glimpse of your baby’s heartbeat, limbs, and movements, but it also establishes your definitive clinical due date.
The Purpose of the Dating Scan
The primary goals of the first-trimester scan are:
- Accurate Gestational Dating: Establishing the true age of the fetus by measuring Crown-Rump Length (CRL).
- Confirming Viability: Visualizing the embryonic heartbeat and blood flow.
- Determining Number of Embryos: Identifying singleton, twin, or higher-order multiple pregnancies.
- Nuchal Translucency (NT) Screening: Assessing fluid thickness at the back of the fetal neck as part of first-trimester genetic screening.
What is Crown-Rump Length (CRL)?
In the first trimester, all human embryos grow at a nearly identical rate regardless of maternal height, weight, or genetics.
The sonographer measures the distance from the top of the head (crown) to the bottom of the torso (rump) in millimeters:
- At 8 weeks, CRL is approximately 16 mm.
- At 10 weeks, CRL is approximately 32 mm.
- At 12 weeks, CRL is approximately 54 mm.
Because growth is so uniform at this stage, CRL allows the dating ultrasound to be accurate within ±3 to 5 days, making it more reliable than menstrual recall.
When Does Your Doctor Change Your Due Date?
If the ultrasound-calculated age matches your LMP date within 5 to 7 days, your doctor will usually keep your original LMP due date.
If the difference exceeds 7 days, your doctor will officially update your Estimated Due Date (EDD) to match the ultrasound date.
What to Do If Your Due Date Changes
If your doctor assigns an updated due date based on your ultrasound, simply toggle to “Estimated Due Date” on our calculator and enter your new date. Our tool will instantly re-calculate all your trimesters and upcoming milestones to match your medical chart!