Due Date Calculator

Estimate when your baby will be born.

Calculation Method

First Day of Last Period

Results

Estimated Due Date

Conception Date --

First Trimester Ends --

Second Trimester Ends --

How Due Date is Calculated

The due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. This assumes a regular 28-day cycle with ovulation occurring on day 14.

If you know your conception date, you can add 266 days (38 weeks) instead, which is the average length of pregnancy from conception.

Frequently Asked Questions

How accurate is the due date?

Only about 5% of babies are born on their exact due date. Most babies arrive within 2 weeks before or after the estimated date, which is completely normal.

What if I have an irregular cycle?

If your cycle is longer or shorter than 28 days, your healthcare provider may adjust your due date. Ultrasound measurements in the first trimester can also help refine the estimate.

When should I see a doctor?

You should schedule your first prenatal appointment as soon as you know you are pregnant, typically around 8 weeks after your last menstrual period.

A due date calculator estimates the day a baby is most likely to arrive, using the date of the last menstrual period (LMP) as the anchor point. The standard method, taught in midwifery and medical training for over 200 years, is called Naegele's Rule: add 280 days (or 40 weeks) to the first day of the LMP. The result is called the Estimated Date of Delivery (EDD), and it is the single date that gets printed on prenatal records and circled on calendars.

The 280-day figure comes from an average human pregnancy length of 40 weeks, measured from the LMP. Conception itself usually happens about 14 days after the LMP in a textbook 28-day cycle, which means the actual fetal age at the EDD is about 38 weeks from conception. This offset is important: when a provider says 'you are 12 weeks pregnant,' the gestational clock started two weeks before conception. The ACOG and NHS both follow this convention, and most pregnancy apps and hospital systems are built on it.

Naegele's Rule assumes a 28-day cycle. For shorter or longer cycles, the due date shifts by the same number of days. A 30-day cycle pushes the EDD 2 days later, a 26-day cycle pulls it 2 days earlier. The formula also assumes that ovulation happened on day 14, which is true on average but not for every cycle. People with irregular cycles, recent hormonal contraceptive use, or uncertain LMP dates will get a less accurate result from LMP-based dating alone.

The most reliable way to refine the due date is an early ultrasound, ideally between 8 and 13 weeks of gestation. Crown-rump length (CRL) measurement at that stage is accurate to within about 5 to 7 days, and many clinicians will adjust the EDD if the ultrasound date differs from the LMP date by more than a week. After 20 weeks, ultrasound estimates become less precise. The CDC, NIH, and March of Dimes all recommend using ultrasound-confirmed dating whenever LMP is uncertain.

How to use

Enter the first day of the last menstrual period (LMP). If the date is not known precisely, use the most accurate estimate available.
Add the average cycle length if it differs from 28 days; the calculator adjusts the due date accordingly.
Optional: enter an ultrasound-based gestational age and date to compare the two estimates side by side.
Submit to see the estimated due date, the current gestational week, and the start and end of each trimester.

Formula

Naegele's Rule: EDD = LMP + 280 days. With cycle length adjustment: EDD = LMP + 280 + (cycle length − 28) days. Ultrasound adjustment: if the first-trimester CRL differs from the LMP-based date by more than 7 days, the ultrasound date is used as the new EDD.

Interpreting your results

The EDD is a midpoint estimate, not a deadline. ACOG defines a 'term' pregnancy as 37 weeks 0 days to 40 weeks 6 days. Delivery between 39 and 40 weeks is 'full term,' and 41 to 42 weeks is 'late term.' A pregnancy that crosses 42 weeks is 'post-term' and usually prompts induction discussions. The CDC reports that about 1 in 12 babies in the US is born preterm (before 37 weeks), so any signs of early labor before 37 weeks warrant immediate medical contact.

Frequently asked questions

What is Naegele's Rule?

Naegele's Rule is the standard obstetric formula for estimating the due date: add 280 days (40 weeks) to the first day of the last menstrual period. It was first described by German obstetrician Franz Karl Naegele in the early 19th century and is still the starting point for pregnancy dating used by ACOG and the NHS.

How accurate is the calculated due date?

Only about 4 to 5 percent of babies are born on the EDD itself. Most arrive within 10 days before or after. An early ultrasound can refine the estimate to within 5 to 7 days. Treat the EDD as a guideline, not a deadline.

Why does cycle length change the due date?

Naegele's Rule assumes ovulation on day 14 of a 28-day cycle. People with longer cycles usually ovulate later, so conception happens later, and the EDD shifts forward. A 30-day cycle moves the EDD about 2 days later, a 32-day cycle about 4 days later, and so on.

When should the ultrasound dating override the LMP?

ACOG recommends using the ultrasound estimate when the first-trimester crown-rump length differs from the LMP-based date by more than 7 days. After 14 weeks, the threshold is closer to 10 days. Once the EDD is set by ultrasound, it generally stays fixed for the rest of the pregnancy.