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Educational tool

Estimate your pregnancy due date and gestational age.

Enter the first day of your last period to get an estimated due date (EDD) and your current gestational age in weeks and days. A dating ultrasound remains the most accurate method.

Medically reviewed by Dr. Shweta Agarwal, MBBS, DGO · Last updated June 2026
Dr. Shweta Agarwal, Founder & Lead Fertility Specialist, at Aansh Hospital & IVF Center, Chandrapur Govt. ART-registered
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How due dates are estimated

40 weeks from the first day of your last period

The estimated due date (EDD) — sometimes called estimated date of delivery or confinement — is conventionally calculated by adding 280 days (40 weeks) to the first day of the last menstrual period. This is known as Naegele's rule, named after the 19th-century obstetrician who described it.

The formula accounts for the fact that gestational age is counted from the LMP, not from conception. Because ovulation typically occurs about 14 days after the LMP in a 28-day cycle, the 280-day figure includes approximately two weeks before conception. If your cycle is not exactly 28 days, this calculator adjusts the EDD to account for the difference.

Important: a first-trimester dating ultrasound (performed between 7 and 13 weeks) is more reliable than an LMP date, especially when cycles are irregular. If your ultrasound due date differs from this calculator's result, discuss with your doctor which date to use.

Calculator

Calculate your estimated due date

The date your most recent period started, before this pregnancy.

If your cycle differs from 28 days, entering the correct length improves the estimate. Range: 21–35.

Dating ultrasound

More accurate than LMP alone

A first-trimester ultrasound measures the crown–rump length (CRL) of the embryo or fetus, which corresponds very closely to gestational age in early pregnancy. When performed between 7 and 13 weeks, it is accurate to within 5–7 days. If your scan EDD differs significantly from this calculator, trust the scan.

Irregular cycles

When LMP is less reliable

Irregular cycles — such as those associated with PCOS or other hormonal conditions — mean ovulation may not have occurred on the assumed day. In these cases, the LMP-based EDD is less reliable, and a dating ultrasound becomes even more important for accurate pregnancy dating and monitoring.

Related resources

Explore further

Medically reviewed by Dr. Shweta Agarwal, MBBS, DGO. Information on this page is educational and does not replace a medical consultation. Outcomes depend on individual clinical factors. This calculator provides an educational estimate of the expected due date and gestational age; it does not constitute medical advice, diagnosis, or treatment. A first-trimester dating ultrasound is the clinically authoritative method for pregnancy dating. Last updated June 2026.

Good to know

Frequently asked questions about pregnancy due dates

How is the due date calculated?
The most widely used method is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This formula assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer than 28 days, the calculator adjusts the estimate accordingly — for example, a 32-day cycle shifts the expected ovulation (and therefore conception) 4 days later, so the EDD is adjusted forward by 4 days. This is still an estimate; a dating ultrasound provides a more accurate EDD by measuring the actual size of the embryo or fetus.
How accurate is an LMP-based due date?
An LMP-based due date is a useful starting estimate, but it has important limitations. It assumes you know the exact start date of your last period, that your cycle is regular, and that ovulation occurred on schedule. Studies show that only about 5% of pregnancies deliver on the calculated due date; most deliver within a window of about two weeks either side. A dating ultrasound performed between 7 and 13 weeks of pregnancy is considered more accurate because it measures the embryo or fetus directly. If there is a significant discrepancy between your LMP date and the ultrasound measurement, the ultrasound date is generally preferred.
What is gestational age?
Gestational age is the conventional measure of how far along a pregnancy is, counted in weeks and days from the first day of the last menstrual period — not from conception. This means gestational age at the point of conception is already approximately two weeks, because ovulation (and therefore fertilisation) typically occurs about two weeks after the LMP. A pregnancy is considered full term at 40 weeks gestational age. When a scan report gives a 'gestational age by scan' it is measuring the size of the fetus and converting it to weeks; it is the same scale.
Why might my ultrasound due date differ from my LMP due date?
Several factors can cause a discrepancy. If ovulation occurred later or earlier than day 14, actual conception was offset from the assumed date. Irregular cycles make LMP-based estimates less reliable. In early pregnancy, crown–rump length (CRL) measured on ultrasound is highly accurate for dating; if the scan-based gestational age differs from LMP by more than 5–7 days in the first trimester, clinicians typically revise the EDD to the scan date. Later in pregnancy, fetal size varies more between individuals, so later scans are less useful for dating. A consultation with Dr. Shweta Agarwal can clarify which date to use based on your specific scan findings.
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