Your due date shapes your entire pregnancy care — and ultrasound is the most accurate tool we have for setting it. Here's how dating scans work.
What Happens at a Dating Scan
Performed ideally between 8 and 13 weeks, the sonographer measures your baby's crown–rump length (CRL) — the distance from head to bottom. Because all embryos grow at a remarkably uniform rate in the first trimester, this single measurement pinpoints gestational age with impressive precision.
The scan also confirms the pregnancy is in the uterus, checks the heartbeat, and counts how many babies you're carrying.
How Accurate Is It?
- First trimester (CRL): accurate to within about ±5–7 days — the gold standard
- Second trimester: accuracy widens to roughly ±10–14 days
- Third trimester: ±3 weeks — far too imprecise for dating, which is why late dating is avoided
This is why an early scan date usually overrides your period-based date: ovulation timing varies widely, but early embryo growth doesn't.
"But I Know My Dates!"
Even with certain conception dates, implantation and early development vary slightly — most guidelines still recommend the CRL-based date when the discrepancy exceeds a few days. It's not doubting you; it's using the most biologically reliable clock.
Why the Due Date Matters
Accurate dating underpins screening test timing, growth assessment later in pregnancy, and decisions about induction — an error early on echoes through everything after.
This article is for general information only and is not a substitute for professional medical advice.
Ready to Book Your Ultrasound Scan?
Mascot Healthcare Clinic, Akoka (Yaba), Lagos offers ultrasound scans by appointment only, Monday–Saturday, 9am–5pm. Book online and choose a time that works for you.