Enter the first day of your last menstrual period to estimate your due date and track your pregnancy progress.
This calculator uses Naegele's rule, the standard method used by healthcare providers:
Due Date = LMP + 280 days (40 weeks)
This calculator provides an estimate only. Your healthcare provider may adjust your due date based on ultrasound measurements or other factors.
A pregnancy due date calculator estimates when your baby is likely to be born based on the first day of your last menstrual period (LMP) or your conception date. The standard calculation assumes a 40-week (280-day) gestation from the LMP, which is called Naegele's rule. This method has been used by obstetricians since the 19th century and remains the foundation of due-date estimation. In reality, only about 5 percent of babies arrive on their exact due date; most are born within a two-week window before or after. Your healthcare provider may adjust the estimate based on ultrasound measurements, especially in the first trimester, when dating is most accurate. The calculator also shows you which trimester you are in, your baby's approximate developmental stage, and key milestone dates throughout your pregnancy.
Naegele’s Rule: Add 280 days (40 weeks) to the first day of your last menstrual period.
Worked example: Last menstrual period started January 15, 2026.
Alternative method: If you know the conception date, add 266 days (38 weeks) instead. Conception typically occurs about 14 days after the first day of your last period in a 28-day cycle.
| Week | Trimester | Key Developments |
|---|---|---|
| Week 4 | 1st | Implantation; positive pregnancy test possible |
| Week 6 | 1st | Heartbeat may be detected on ultrasound |
| Week 8 | 1st | All major organs forming; embryo is 1.6 cm |
| Week 10 | 1st | Fingers and toes develop; embryo becomes a fetus |
| Week 12 | 1st | End of first trimester; risk of miscarriage drops significantly |
| Week 16 | 2nd | Baby can make facial expressions; gender may be visible |
| Week 20 | 2nd | Halfway point; anatomy scan ultrasound; baby is ~25 cm |
| Week 24 | 2nd | Viability milestone; lungs developing surfactant |
| Week 28 | 3rd | Third trimester begins; baby opens eyes |
| Week 32 | 3rd | Baby weighs ~1.8 kg; practices breathing movements |
| Week 36 | 3rd | Baby moves into head-down position; considered early term |
| Week 37 | 3rd | Full term begins; lungs are mature |
| Week 39 | 3rd | Ideal delivery window begins |
| Week 40 | 3rd | Official due date; induction may be discussed if overdue |
Naegele's rule is accurate to within about two weeks for most pregnancies. First-trimester ultrasounds can narrow this to plus or minus 5 days. Only about 5% of babies are born on their exact due date, but about 80% arrive within 2 weeks of it.
If your cycle is regularly longer or shorter than 28 days, your ovulation date shifts accordingly. A 35-day cycle means you likely ovulated around day 21 rather than day 14. Adjust by adding the extra days to your due date. Your doctor may use ultrasound measurements instead for accuracy.
The most accurate dating ultrasound is performed between weeks 8 and 13. At this stage, embryos grow at very consistent rates, so the crown-rump length measurement can predict the due date within 3 to 5 days. Later ultrasounds are less reliable for dating because growth rates vary more between babies.
A pregnancy is considered early term at 37 weeks, full term at 39 weeks, late term at 41 weeks, and post-term at 42 weeks. Babies born at 39 to 40 weeks generally have the best health outcomes. Elective deliveries before 39 weeks are discouraged unless medically necessary.
Yes. If an early ultrasound shows a significant discrepancy (more than 7 days) from the LMP-based date, your provider will likely adjust the due date. Later changes are less common but can occur with revised ultrasound measurements.
Pregnancy weeks are counted from the first day of your last menstrual period, not from conception. This means that during weeks 1 and 2, you are technically not yet pregnant. Conception typically happens around week 2, and the embryo implants around week 3 to 4.
Pregnancy Due Date Calculator - Estimate Your Due Date is one of the most searched-for tools on the internet, and for good reason. Whether you are a student, professional, or just someone trying to solve an everyday problem, having a reliable pregnancy due date - estimate your due date tool at your fingertips saves time and reduces errors. This calculator handles all the common scenarios you might encounter, from simple calculations to more complex multi-step problems. The mathematics behind pregnancy due date - estimate your due date calculations has been refined over centuries, with practical applications spanning education, business, science, engineering, healthcare, and daily life. Understanding how the calculation works — not just plugging in numbers — gives you the confidence to verify results and catch mistakes. In this comprehensive guide, we will walk through the formulas, show you worked examples, provide reference tables, and answer the most common questions people ask about pregnancy due date - estimate your due date calculations.
Determine what values you have and what you need to find. For pregnancy due date - estimate your due date calculations, clearly identify each input value and its unit.
Use the appropriate formula for your specific pregnancy due date - estimate your due date calculation. Enter your values carefully, paying attention to units and decimal places.
Perform the calculation step by step. If doing it by hand, work through each operation in order. Or use this calculator for instant, accurate results.
Check that your answer makes sense in context. A good practice is to estimate the result mentally first, then compare with the calculated answer.
| Scenario | Result |
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| Example 1 | Use calculator above |
| Example 2 | Use calculator above |
| Example 3 | Use calculator above |
| Example 4 | Use calculator above |
| Example 5 | Use calculator above |
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| Example 10 | Use calculator above |
Pregnancy Due Date - Estimate Your Due Date measurements are used extensively in healthcare, fitness, and wellness. Doctors and nurses use these calculations for patient assessments, treatment planning, and monitoring progress. Nutritionists and dietitians rely on pregnancy due date - estimate your due date calculations to create meal plans, assess nutritional status, and set health goals. Fitness trainers use them to design workout programs, track client progress, and adjust training intensity. Public health researchers use pregnancy due date - estimate your due date data at the population level to identify trends, evaluate interventions, and set policy recommendations. Understanding your own pregnancy due date - estimate your due date numbers empowers you to take control of your health and have more informed conversations with healthcare providers.
Always double-check your inputs before calculating. A small error in the input can lead to a significantly wrong result. When working with pregnancy due date - estimate your due date calculations, it helps to estimate the expected result first — if your calculated answer is wildly different from your estimate, you probably made an input error. Also, be careful with units: mixing up meters and centimeters, or dollars and cents, is one of the most common calculation mistakes.
The concept behind pregnancy due date - estimate your due date has been used by humans for thousands of years. Ancient civilizations like the Egyptians, Babylonians, and Greeks all developed methods for these types of calculations, often using remarkably clever shortcuts that are still useful today.
Enter your values in the input fields above and click Calculate (or the result updates automatically as you type). The calculator will show you the result instantly along with a breakdown of the calculation.
Yes, this calculator is completely free to use with no sign-up required. Use it as many times as you need.
This calculator uses standard mathematical formulas and is accurate to multiple decimal places. Results are rounded for readability but the underlying calculations use full precision.
Yes, this calculator is fully responsive and works on all devices including smartphones, tablets, and desktop computers.
The calculator uses standard mathematical formulas for pregnancy due date - estimate your due date calculations. The specific formula is explained in the "How to calculate" section above.
Pregnancy Due Date - Estimate Your Due Date calculations come up frequently in everyday life, from shopping and cooking to finance and professional work. A calculator ensures accuracy and saves time on complex calculations.
Simple pregnancy due date - estimate your due date calculations can be done mentally using shortcuts described in our guide above. For complex calculations or when accuracy matters, use this calculator.
The most common mistakes are: entering wrong values, mixing up units, forgetting to convert between different formats, and rounding too early in multi-step calculations.
Pregnancy Due Date - Estimate Your Due Date calculations are widely used in business for financial analysis, planning, budgeting, pricing, and decision-making. See our "Industry applications" section above for details.
Our guide above covers the fundamentals. For more advanced topics, check out Khan Academy, Coursera, or your local library for pregnancy due date - estimate your due date-related educational resources.
Yes, this calculator handles numbers of any practical size. JavaScript can accurately represent integers up to 2^53 (about 9 quadrillion) and decimals to about 15-17 significant digits.
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