Due Date Estimation by Ultrasound
Ultrasound dating is considered the most accurate method for estimating a due date, especially when performed in the first trimester (before 14 weeks). The ultrasound measures the embryo or fetus to determine gestational age.
How Does It Work?
During an ultrasound, your healthcare provider measures the size of the embryo (crown-rump length in early pregnancy) or fetal head (biparietal diameter later on). These measurements correspond to specific gestational ages. The calculator works backwards from the ultrasound date and gestational age to determine an estimated LMP, then adds 280 days.
When Is Ultrasound Dating Most Accurate?
- Weeks 6-9: Accurate within ± 3-5 days.
- Weeks 9-14: Accurate within ± 5-7 days.
- Weeks 14-20: Accurate within ± 7-10 days.
- After 20 weeks: Less reliable for dating due to natural size variation.
When Should the Due Date Be Changed?
According to ACOG guidelines, a due date established by first-trimester ultrasound should only be changed if subsequent measurements differ by more than 7-10 days. Your healthcare provider will determine the most appropriate due date for your pregnancy.