What this calculator does
An early ultrasound is the most accurate way to date a pregnancy. This tool takes the date of the scan and the gestational age (GA) the sonographer measured at that scan — for example "8 weeks 3 days" — and works out the estimated due date (EDD) plus how many weeks and days pregnant you are today. Dating is based on the standard 280-day (40-week) full-term gestation.
How to use it
Enter the scan date, the gestational age recorded at the scan (weeks and days), and today's date. The calculator returns the EDD as a calendar date and your current gestational age in weeks + days.
The formula
First the scan GA is converted to days: \(\text{GA}_{\text{scan}} = \text{weeks} \times 7 + \text{days}\). The days still needed to reach a full 280-day term is \(280 - \text{GA}_{\text{scan}}\), and adding that to the scan date gives the EDD. Current gestational age is the scan GA plus the number of days elapsed between the scan date and today.
$$\text{EDD} = \text{Scan Date} + \left(280 - g\right)\text{ days}$$$$\text{where}\quad \left\{ \begin{aligned} g &= 7 \times \text{GA Weeks} + \text{GA Days} \\ \text{Scan Date} &= \left(\text{Y},\ \text{M},\ \text{D}\right) \end{aligned} \right.$$
Worked example
Scan on 2024-01-15 measuring 8 weeks 3 days.
$$\text{GA}_{\text{scan}} = 8 \times 7 + 3 = 59\text{ days}$$$$\text{Days to term} = 280 - 59 = 221$$Adding 221 days to 2024-01-15 gives an EDD of 2024-08-23. If today is the scan date, current GA is still 8w 3d (59 days).
FAQ
Is ultrasound dating better than LMP dating? Yes — first-trimester ultrasound is generally more accurate than dating from the last menstrual period, especially with irregular cycles.
What is the 280-day rule? A full-term pregnancy is conventionally counted as 280 days (40 weeks) of gestational age.
Will my real due date change? Clinicians may adjust dating after later scans; this calculator reflects the single scan you enter and is for information only, not medical advice.
