Free Pregnancy Due Date Calculator

Estimate your baby's arrival date using your last menstrual period or conception date. See your trimesters, key milestones, and current week of pregnancy — free, no signup required.

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Enter Your Details
Enter the first day of your last period.

Or use conception date instead

If known, leave LMP blank and enter your conception date. We will subtract 14 days to estimate your LMP.

Enter your LMP or conception date above to calculate your estimated due date.

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How Pregnancy Due Dates Are Calculated

The most widely used method for estimating a pregnancy due date is Naegele's rule, developed by German obstetrician Franz Karl Naegele in the early 19th century. The rule states that the estimated due date (EDD) is calculated by taking the first day of the last menstrual period (LMP), adding one calendar year, subtracting three months, and adding seven days. In simpler terms, this works out to adding 280 days (40 weeks) to the first day of the LMP.

The 280-day figure is based on the assumption that ovulation occurs on day 14 of a 28-day cycle, making conception approximately 14 days after the LMP. The remaining 266 days (38 weeks) represent the average duration of human gestation from conception to birth. This calculator adjusts for cycle lengths other than 28 days: if your cycle is longer, ovulation likely occurs later, so we add the difference to the standard 280 days. For example, a 35-day cycle shifts the EDD forward by 7 days.

If you know your conception date — perhaps from IVF records or a fertility monitor — you can enter that instead. The calculator subtracts 14 days to approximate your LMP and then applies the same 280-day calculation. Ultrasound dating during the first trimester (typically the 8–12 week scan) remains the most accurate method and will be used by your healthcare provider to confirm or revise your due date.

Understanding Your Trimesters

Pregnancy is conventionally divided into three trimesters, each spanning roughly 13 weeks and characterised by distinct stages of fetal development and maternal experience.

First trimester (weeks 1–13): This is the period of greatest organogenesis — the development of the baby's major organs and systems. The heart begins beating around week 6. By the end of the first trimester, the fetus has fully formed fingers and toes, functioning kidneys, and visible facial features. For the mother, this trimester is often marked by nausea, fatigue, breast tenderness, and frequent urination. The risk of miscarriage is highest in the first trimester, which is why many parents wait until week 12 or 13 to share the news widely.

Second trimester (weeks 14–27): Often called the "honeymoon trimester," this period typically brings relief from early pregnancy symptoms. The baby's movements (quickening) become perceptible, usually between weeks 16 and 25. Major developmental milestones include the formation of fingerprints (week 17), the ability to hear sounds (week 18), and the opening of the eyes (week 28). The 20-week anatomy ultrasound scan is a critical appointment during this trimester, checking fetal anatomy and growth in detail.

Third trimester (weeks 28–40+): The final stretch involves rapid fetal growth and weight gain. The baby's lungs mature — a key milestone, as lung development is what determines viability outside the womb. Babies born after 37 weeks are considered full-term. During this trimester, you will have more frequent prenatal appointments, and your provider will check the baby's position, your blood pressure, and signs of conditions such as pre-eclampsia.

Important Pregnancy Milestones

While every pregnancy is unique, certain appointments and tests are recommended at specific points in gestation:

20-week anatomy scan: This detailed ultrasound checks all major organ systems, measures fetal growth, examines the placenta and amniotic fluid, and can reveal structural abnormalities. It is also the appointment where many parents find out the baby's sex if they wish to know.

28-week glucose challenge test: Gestational diabetes screening typically takes place between weeks 24 and 28. You drink a glucose solution and have your blood sugar measured one hour later. If the result is elevated, a longer three-hour glucose tolerance test is performed to confirm or rule out gestational diabetes.

36-week weekly appointments: From week 36 onward, most providers schedule weekly check-ups. These visits include checks for Group B Streptococcus (GBS), assessment of fetal position, cervical changes, and blood pressure monitoring. This increased frequency allows your care team to catch any late-developing complications early.

Other notable milestones include the first trimester combined screening (weeks 11–13), quad screen or anatomy scan (weeks 15–20), and the rhogam injection for Rh-negative mothers at week 28. Your specific care plan will be tailored to your health history and risk profile.

Frequently Asked Questions

This calculator applies Naegele's rule, the same formula used clinically, adjusted for your cycle length. As a general estimate it is accurate to within a week or two, but only about 4% of babies are born on their exact due date. First-trimester ultrasound dating is more accurate and is used by your healthcare provider to confirm the EDD. Factors such as irregular cycles, unknown conception date, and individual variation in gestation length all affect accuracy.

Naegele's rule is the standard clinical formula for estimating a pregnancy due date. It adds 280 days (40 weeks) to the first day of the last menstrual period. The 280-day figure assumes a 28-day cycle with ovulation on day 14, giving 14 days from LMP to conception and 266 days (38 weeks) from conception to delivery. The rule was proposed by Franz Karl Naegele in 1812 and remains the foundation for obstetric dating today, though it is routinely verified or corrected by ultrasound.

Yes. Your provider may revise your due date based on first-trimester ultrasound measurements, which are the most accurate dating method available. If the ultrasound-based date differs from your LMP-based date by more than 5–7 days in the first trimester (or more than 10–14 days in the second trimester), the ultrasound date is typically used. Later ultrasounds are less reliable for dating as fetal growth becomes more variable.

If your cycles are irregular or significantly longer or shorter than 28 days, LMP-based dating is less reliable. You can still use this calculator with your best estimate of cycle length to get a rough date, but ultrasound dating will be much more informative. If you tracked ovulation using an LH surge monitor or basal body temperature, your ovulation date is a better starting point than the LMP — enter your conception date instead.

The first trimester runs from week 1 through the end of week 13 (the first 13 weeks of pregnancy). The second trimester spans weeks 14 through 27. The third trimester begins at week 28 and continues until delivery, which is considered full-term at 37–42 weeks. Some sources define the trimesters slightly differently — for example, dividing by months rather than weeks — but the 13/14/28 week boundaries are the most commonly used in clinical practice.

The American College of Obstetricians and Gynecologists (ACOG) defines full-term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37–38 weeks, late term is 41 weeks, and post-term is 42 weeks or beyond. Babies born before 37 weeks are preterm. Research shows that outcomes are best for babies born at 39–40 weeks, as the final weeks allow critical lung maturation and brain development. Your provider will discuss induction or monitoring if you go past 41 weeks.

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