Figuring out when a baby is due usually starts with one question: do you go by the last period or by the ultrasound? This EDD calculator lets you run both methods side by side, see the gestational age each one produces, and get a single recommended due date instead of guessing which number to trust.

How to use this EDD calculator

1. Under the LMP method, enter the “Current Date” (today’s date, or the date you want the calculation based on).
2. Enter “Your Last Period” date, meaning the first day of your last menstrual period.
3. The calculator immediately shows the “LMP EDD”, your estimated due date based on a standard 280-day pregnancy length from that period.
4. Under the Ultrasound method, enter the “Date of the Ultrasound” (the day the scan was performed).
5. Enter “Ultrasound Weeks” and “Ultrasound Days”, the gestational age the sonographer measured at that scan (for example, 8 weeks 4 days).
6. Enter your “Menstrual Cycle Length” in days, using the plus and minus buttons to adjust from the default if your cycle is shorter or longer than 28 days.
7. The calculator generates the “Ultrasound EDD” based on the scan measurement and your cycle length.
8. Review the gestational age display, which shows the LMP-based age, the ultrasound-based age, and the difference between the two, along with a recommended due date that reflects which method is more reliable given how far along you are.
9. Use “Save as Image” or “Download PDF” if you want a copy of the results, or check “Saved Calculations” to revisit past entries. “Clear All” resets the history.

What this EDD calculator does

EDD stands for Estimated Date of Delivery, sometimes called the due date. This EDD calculator works out that date two separate ways and then tells you which one to lean on. The LMP method counts forward from the first day of your last period, the traditional way due dates have been calculated for decades. The ultrasound method starts from a scan measurement, since a sonographer measures the size of the fetus and estimates gestational age from that, which is then converted into its own due date.

Most pregnant people know these two numbers can disagree, sometimes by a week or more, especially if periods are irregular or cycle length isn’t exactly 28 days. Rather than forcing you to pick one, this calculator shows both, quantifies the gap between them, and applies redating logic so you get a single practical answer for your due date.

The two calculation methods explained

LMP method

This is Naegele’s rule in practice: add 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle with ovulation on day 14.

This method is quick and doesn’t require a scan, but it assumes regular ovulation timing. If your cycle runs longer or shorter than 28 days, the true conception date shifts, which is why the calculator also lets you dial in cycle length on the ultrasound side.

Ultrasound method

Here the calculator works backward from a measured gestational age at a known scan date. If the ultrasound showed you were, say, 12 weeks 3 days along on a specific date, the calculator subtracts that gestational age from 40 weeks to find how many days remain, then adds that to the scan date.

Cycle length is factored in because ultrasound gestational age formulas are typically built around a standard 28-day cycle, so entering your actual cycle length lets the calculator adjust for cycles that run longer or shorter.

Comparing the two methods

The calculator displays both estimated due dates plus the difference between the two gestational ages, so you can see at a glance whether they agree closely or point to different weeks.

Why the ultrasound date can override the LMP date

This EDD calculator follows the redating guidance referenced from ACOG Committee Opinion No. 700 (2017): a first-trimester ultrasound is the most reliable single measurement for establishing gestational age, because early fetal growth is remarkably consistent across pregnancies. Scans done later in pregnancy reflect the baby’s individual growth pattern rather than precise dating, so they carry less weight for redating purposes. That’s why the calculator specifically prompts for a first-trimester scan when comparing methods, and why a due date pinned to that scan is often considered more accurate than one based purely on LMP, particularly if your cycles are irregular.

A few things worth knowing

• Only about 5% of babies are actually born on their exact estimated due date, the EDD is a target, not a guarantee.
• If you’re unsure of your LMP date, the ultrasound method is generally the more dependable of the two.
• A cycle length that differs from 28 days changes when ovulation likely occurred, which shifts the LMP-based calculation even if the period date itself is remembered correctly.
• A gap of more than about a week between the LMP EDD and ultrasound EDD is one of the main reasons care providers will redate a pregnancy based on the scan.

This EDD calculator gives you a fast, side-by-side estimate using two well-established methods, but it produces an estimate only, not medical advice. Your actual due date and any decision to redate a pregnancy should be confirmed by your doctor or midwife, who can factor in your full history and scan reports.

Frequently asked questions