Published by MetaSys Limited · Page updated 19 September 2026 · Editorial policy. Educational information; not a medical review.

How This Due Date Calculator Works

Finding out you're pregnant almost always leads to the same first question: when will the baby actually arrive? This calculator is designed to give you a fast, evidence-based estimate using the same method most clinics use at your very first prenatal visit, before any ultrasound has been performed. It won't tell you the exact day your baby will be born — nothing can do that reliably — but it will give you a scientifically grounded target date and a sense of where you currently stand in the pregnancy timeline.

Naegele's Rule: The Formula Behind the Estimate

This calculator uses Naegele's Rule, a method developed in the early 1800s by German obstetrician Franz Naegele and still the standard starting point in obstetric care today. The rule is straightforward: take the first day of your last menstrual period (LMP) and add 280 days, which is the same as 40 weeks. That 280-day figure comes from a textbook 28-day menstrual cycle with ovulation occurring on day 14 — pregnancy is dated from the start of that cycle even though conception itself happens roughly two weeks later, because most people can pinpoint their last period with far more confidence than their exact day of conception. If you happen to know your conception date instead, perhaps from ovulation tracking or fertility treatment, this calculator's alternate mode adds 266 days to that date, which lands on the same estimated due date for a standard 28-day cycle.

Reading Your Current Week and Trimester

Alongside your due date, this calculator estimates how far along you are right now by counting the days between your last period and today, then dividing by seven. Pregnancy is traditionally divided into three trimesters: the first trimester spans weeks 1 through 13, covering the earliest and often most delicate stage of fetal development; the second trimester runs from week 14 through week 27, frequently described as the most comfortable stretch for many people; and the third trimester begins at week 28 and continues until birth, typically around week 40. Each trimester carries its own set of typical symptoms, recommended screenings, and developmental milestones, so knowing which one you're in helps you anticipate what your provider will likely discuss at your next appointment.

Using This Estimate Practically

Treat the date this calculator produces as the center of a range rather than a fixed deadline circled on a calendar. Delivery can occur before or after the estimated date. “Full term” has a specific clinical meaning, and your maternity team should explain the relevant timing and any monitoring needed. Many people find it more useful to think in terms of a due month or due window when discussing timelines with family, employers, or for planning parental leave, rather than fixating on a single calendar square.

Important Caveats to Keep in Mind

Naegele's Rule assumes a standard 28-day cycle with ovulation on day 14. If your cycles run consistently longer or shorter, or are irregular, the actual conception date can shift by a week or more in either direction, which throws off an LMP-based estimate accordingly. This is exactly why an ultrasound-confirmed due date from a healthcare provider, especially one performed in the first trimester when fetal measurements are most predictable, is considered more accurate than LMP-based dating alone. This calculator is a helpful planning tool, not a diagnostic instrument — always confirm your due date and track your pregnancy progress with a qualified healthcare provider.

Read Methodology & Sources and Calculator Limitations. Report an error.

Pregnancy Due Date Calculator — Frequently Asked Questions

How accurate is a due date calculated from my last period?

It's a solid estimate, not a guarantee. Naegele's Rule assumes a textbook 28-day cycle with ovulation on day 14, so only about 5% of babies are actually born on their exact calculated due date. Most deliveries happen within a window of about two weeks before to one week after the estimated date. An ultrasound measurement in the first trimester is generally considered more accurate than LMP-based dating alone, especially if your cycles are irregular.

What if I don't know my last menstrual period date?

If you know roughly when conception occurred, such as from ovulation tracking or a single instance of intercourse, switch this calculator to Conception Date mode, which adds 266 days instead of 280. Otherwise, your healthcare provider can estimate gestational age from a first-trimester ultrasound, which measures the size of the fetus and is often more reliable than memory of your last period.

Why does Naegele's Rule add exactly 280 days?

A full-term pregnancy is measured as 40 weeks from the first day of your last menstrual period, and 40 weeks multiplied by 7 days equals 280 days. This counts from LMP rather than conception because most people know their last period date with more confidence than their exact conception date, even though actual fertilization typically occurs about two weeks later.

Does an irregular menstrual cycle affect the accuracy of this calculation?

Yes, significantly. Naegele's Rule assumes ovulation occurs on day 14 of a 28-day cycle. If your cycles are consistently longer or shorter than 28 days, or irregular in length, the actual conception date can shift by a week or more in either direction, which throws off the LMP-based estimate. In these cases, an early ultrasound is considerably more reliable than LMP-based dating.

What are the three trimesters and why do they matter?

The first trimester runs from week 1 through week 13, the second from week 14 through week 27, and the third from week 28 until birth, typically around week 40. Each trimester is associated with distinct stages of fetal development and different screening tests, symptoms, and prenatal care milestones, which is why knowing your current week and trimester helps you and your provider anticipate what comes next.

Pregnancy Due Date Calculator