Due Date Calculator — from an Ultrasound Scan
A first-trimester ultrasound is the most accurate way to date a pregnancy. If your scan report gives a gestational age — for example '12 weeks 3 days' — enter that along with the date of the scan, and this calculator projects your estimated due date and the rest of your pregnancy timeline. Early dating scans (using crown-rump length before 14 weeks) are the gold standard doctors use to set or revise the EDD.
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Frequently asked questions
How is the due date calculated from an ultrasound?+
The scan measures the baby and reports a gestational age (weeks and days). The calculator subtracts that age from the scan date to find the effective start of pregnancy, then adds 40 weeks for the estimated due date. An early scan (before 14 weeks) is the most reliable.
Why did my ultrasound due date differ from my LMP due date?+
LMP dating assumes a regular 28-day cycle and ovulation on day 14, which isn't true for everyone. If ovulation was earlier or later, the scan-based date — which measures the baby directly — is more accurate, and doctors will usually use the ultrasound date when the difference is more than about 5–7 days in the first trimester.
How is the pregnancy due date calculated?+
The standard estimated due date (EDD) is 40 weeks (280 days) from the first day of your last menstrual period (LMP) using Naegele's rule. If you know your conception date, IVF transfer date, or a scan-dated gestational age instead, this calculator works backwards from those to the same EDD. Only about 5% of babies are born on their exact due date — most arrive within two weeks either side.
What is EDD in pregnancy?+
EDD stands for Estimated Due Date — the date your baby is expected to arrive, calculated as 40 weeks from your last menstrual period. It is an estimate, not a deadline: a normal, full-term birth can happen any time from 37 to 42 weeks.
Which dating method is the most accurate?+
A first-trimester ultrasound (crown-rump length, ideally before 14 weeks) is the most accurate way to date a pregnancy and is what doctors use to confirm or revise the due date. LMP dating is reliable only if your cycles are regular and you remember the date. For IVF, the transfer date gives a very precise due date because the day of fertilisation is known.
When does each trimester start and end?+
First trimester: weeks 1–12. Second trimester: weeks 13–26. Third trimester: weeks 27–40. This calculator shows your current trimester, your week-by-week progress, and key appointment milestones such as the NT scan (11–13 weeks) and anomaly scan (around 20 weeks).
How many weeks pregnant am I right now?+
Gestational age is counted from the first day of your last period, not from conception — so you are already counted as about 2 weeks pregnant at the moment of conception. Enter your date above and the calculator shows your current weeks and days, your trimester, and how many weeks remain.