Ultrasound Accuracy at 16 Weeks: Margin of Error & What to Expect

How Accurate Is a 16‑Week Ultrasound for Dating?

Ultrasounds performed in the first trimester (before 14 weeks) are the most accurate for determining gestational age. At 16 weeks and 2 days, an ultrasound remains quite reliable, with a typical margin of error of 5–7 days. This means the estimated due date based on this scan is unlikely to be off by more than a week.

Factors That Can Affect Accuracy

• Fetal growth variation – Some babies grow faster or slower than average.
• Maternal factors – Obesity or uterine anomalies can limit image clarity.
• Technician skill and equipment – As with earlier scans, expertise and machine quality matter.

Next Steps

Discuss the results with your healthcare provider. They will correlate the ultrasound findings with your menstrual history and any prior scans to give you the most accurate due date estimate.