Calculate your estimated due date, conception window, trimester dates, and key pregnancy milestones. Based on Naegele\'s Rule with cycle length adjustment.
🎀 Estimated Due Date
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Conception Window
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First Trimester Ends
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Second Trimester Ends
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Third Trimester Starts
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Key Pregnancy Milestones
How Due Dates Are Calculated: Understanding Naegele\'s Rule
The most common method for calculating a pregnancy due date is Naegele\'s Rule, developed by German obstetrician Franz Naegele in the early 1800s. His formula adds 280 days (40 weeks) to the first day of your last menstrual period (LMP), then subtracts 3 months and adds 7 days. For example, if your LMP was March 15: add 7 days = March 22, subtract 3 months = December 22 estimated due date.
Cycle Length Matters
Naegele\'s Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is 35 days, ovulation occurs around day 21 — 7 days later — so your due date shifts forward by 7 days. Our calculator automatically adjusts for your cycle length. For the most accurate dating, your healthcare provider will use a first-trimester ultrasound, which is accurate within 5-7 days.
Important Disclaimer
This calculator provides an estimate only and is not a substitute for professional medical advice. Always consult your doctor, midwife, or OB-GYN for personalized pregnancy care and accurate dating via ultrasound. Every pregnancy is unique, and only your healthcare provider can give you an exact due date based on your individual circumstances.
Frequently Asked Questions
The standard method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). For example, if LMP was January 1, the estimated due date is October 8. This assumes a 28-day cycle with ovulation on day 14. Our calculator adjusts for your actual cycle length.
Only about 4% of babies are born exactly on their due date. Most (80%) arrive within 2 weeks before or after. The due date is an estimate — your healthcare provider may adjust it based on ultrasound measurements, which are most accurate in the first trimester (within 5-7 days).
If your cycle is longer than 28 days, ovulation occurs later and your due date shifts. For a 32-day cycle, add 4 extra days. Our calculator automatically adjusts the due date based on your cycle length. Irregular cycles make the estimate less reliable — an early ultrasound provides the most accurate dating.
First trimester: Week 1 to Week 13 (months 1-3). Second trimester: Week 14 to Week 27 (months 4-6). Third trimester: Week 28 to Week 40+ (months 7-9). Each trimester has distinct developmental milestones and screening tests. Our calculator shows your exact trimester dates.
An ultrasound can typically determine sex around 18-20 weeks (the anatomy scan). Non-invasive prenatal testing (NIPT) can detect sex as early as 10 weeks via a blood test. Our calculator shows when you reach these key milestones.
First-time moms typically feel movement (quickening) between 18-22 weeks. Experienced moms may feel it as early as 16-18 weeks. If you have not felt movement by 24 weeks, contact your healthcare provider.
Pregnancy is dated from the first day of your last period, not from conception. So at "4 weeks pregnant," conception actually occurred about 2 weeks ago. This means weeks 1-2 of pregnancy occur before you are actually pregnant. The total is 40 weeks from LMP to due date.
Yes. Add 266 days (38 weeks) to the conception date. This is slightly more accurate than the LMP method, but few women know their exact conception date. IVF patients know the exact embryo transfer date, making this the most precise dating method.
Early term: 37 weeks 0 days to 38 weeks 6 days. Full term: 39 weeks 0 days to 40 weeks 6 days. Late term: 41 weeks 0 days to 41 weeks 6 days. Postterm: 42 weeks and beyond. Babies born at 39-40 weeks have the best health outcomes.