Calculate your estimated due date from ultrasound measurements. Enter scan date and gestational age to find your EDD accurately.
An ultrasound-based due date is considered the most clinically accurate way to estimate when your baby will arrive, especially when performed in the first trimester. This calculator takes the date of your ultrasound and the gestational age measured by the sonographer to calculate your estimated due date.
During an early ultrasound, the sonographer measures the crown-rump length (CRL) of the embryo or fetus and converts it to gestational age. This measurement is remarkably precise in the first trimester — accurate to within 3-5 days. The calculator then works backward and forward: it determines the equivalent LMP date and adds 280 days to find the EDD.
If you have both an LMP-based date and an ultrasound date, providers generally use the ultrasound date when it differs by more than 7 days in the first trimester, more than 10 days in the second trimester, or more than 14 days in the third trimester.
Ultrasound dating is the gold standard in obstetric care when LMP is uncertain or unavailable. It provides an evidence-based EDD that improves the accuracy of prenatal screening timing, growth monitoring, and delivery planning. Using this calculator after your dating scan ensures all your pregnancy planning is based on the most reliable information.
EDD = Scan Date + (280 − GA_at_scan_in_days) Where: GA_at_scan_in_days = (weeks × 7) + days LMP Equivalent = Scan Date − GA_at_scan_in_days 280 = total days of a full-term pregnancy from LMP
Result: October 8, 2026
An ultrasound on March 15, 2026 showing 10 weeks 3 days gestational age means the LMP equivalent is January 1, 2026 (March 15 minus 73 days). Adding 280 days to that LMP gives an EDD of October 8, 2026. The remaining pregnancy is 280 − 73 = 207 days from the scan date.
A dating scan performed between 8-13 weeks has a margin of error of just 3-5 days. By the second trimester, accuracy drops to 7-10 days, and by the third trimester it can be off by 2-3 weeks. This is because early embryonic growth is remarkably consistent, while later fetal growth varies with genetics, nutrition, and other factors.
ACOG and SMFM guidelines specify clear thresholds for when an ultrasound-based date takes precedence over LMP. In most clinical settings, the best estimate from the earliest ultrasound is used for the official EDD. If multiple early ultrasounds are performed, the one with the closest agreement to LMP is preferred.
Your ultrasound report lists gestational age in weeks and days (e.g., 10w3d). Enter these values into the calculator along with the scan date. The tool will compute your EDD and all trimester milestones. Keep a copy of your ultrasound report for reference at all future prenatal appointments.
LMP dating assumes a 28-day cycle with ovulation on day 14, which may not apply. Ultrasound directly measures the embryo or fetus, removing assumptions about cycle length and ovulation timing. In the first trimester, embryos grow at nearly identical rates, making CRL an excellent age estimate.
In the first trimester, the sonographer measures crown-rump length (CRL) — the distance from the top of the head to the rump. Published growth charts convert CRL to gestational age. After 14 weeks, head circumference, femur length, and abdominal circumference are used.
ACOG guidelines recommend using the ultrasound date if the discrepancy exceeds 5 days (before 9 weeks), 7 days (9-14 weeks), 10 days (14-16 weeks), or 14 days (16-22 weeks). Your provider will decide which date to use.
Generally, due dates are not changed after the first trimester unless there was no early dating. Later ultrasounds are less accurate for dating because fetal growth rates vary more. A size discrepancy later in pregnancy is more likely to indicate growth variation than wrong dating.
Gestational age measures pregnancy from the first day of the LMP, not from conception. It includes about 2 weeks before the egg was fertilized. So at 8 weeks gestational age, the embryo has been developing for approximately 6 weeks.
Yes, first-trimester ultrasound dating is accurate for twin pregnancies. Each twin is measured individually. Note that twins often deliver earlier than singletons, so the EDD represents a full-term singleton timeline.
If the gestational age is less than 6 weeks, it may be too early to detect a heartbeat. Your provider will likely schedule a follow-up scan in 1-2 weeks. A heartbeat is typically visible by 6-7 weeks gestational age.
A transvaginal ultrasound can detect a gestational sac as early as 4.5-5 weeks. A yolk sac appears around 5-5.5 weeks, and an embryo with heartbeat by 6-7 weeks. An abdominal ultrasound may require an additional week to visualize these structures.