💪 Health & Fitness Calculators › Pregnancy & Due Date

Pregnancy Due Date Calculator (Naegele's Rule)

Estimate your baby's due date from the last menstrual period or conception date using Naegele's rule, with gestational age, trimester timeline, and cycle-length adjustment.

Due Date Calculator — interactive calculator, computed instantly in your browser.

📚 Authoritative Educational Guide

Naegele's Rule: How Your Pregnancy Due Date Is Estimated

👶 Gestational Timeline

280 Days From the Last Menstrual Period

LMP — Day 0 week 0 begins Week 14 2nd trimester Week 28 3rd tri Day 280 = 40 wks due = LMP + 280 days · conception method: conception + 266 days Cycle ≠ 28? shift the date by (cycle − 28) days — longer cycles mean later ovulation. Only ≈ 5% of babies arrive on the exact due date; ~90% are born between 37–42 weeks.

Key Takeaway: The due date is a 40-week estimate anchored on the first day of the last period — a planning landmark, not an appointment.

A pregnancy due date (estimated date of delivery, EDD) is the single number every scan, test, and appointment orbits around. Obstetrics counts gestation from the first day of the last menstrual period (LMP) — roughly two weeks before conception — which makes the standard math elegantly simple: LMP + 280 days = 40 weeks. This is Naegele's rule, formulated in 1812, and it still anchors nearly every modern pregnancy calculator.

1. Naegele's Rule, Step by Step

EDD = LMP − 3 months + 1 year + 7 days = LMP + 280 days
  • Take the LMP date, subtract 3 calendar months, add 1 year, then add 7 days — arithmetically identical to +280 days
  • Cycle adjustment: ovulation in a 32-day cycle arrives 4 days later → shift the due date +4 days (cycle − 28)
  • Conception method: fertilization happens ~14 days after LMP, so conception + 266 days lands on the same date
  • Gestational age always runs ~2 weeks ahead of fetal (embryonic) age — scans after 14 weeks confirm dating

2. What the Trimester Timeline Looks Like

The 40-week clock splits into trimesters: first trimester weeks 0–13 (organ formation, first ultrasound and bloodwork), second trimester weeks 14–27 (anatomy scan around week 20, movement felt), and third trimester weeks 28–40 (growth checks, group-B strep test at 36–37 weeks, birth preparation). Full term is 39–41 weeks, while 37–42 weeks is the accepted term window — only about 5% of births land on the exact due date, and roughly 90% arrive within the 37–42 week band.

3. LMP vs Conception vs Ultrasound Dating

  1. LMP method: free and instant, but assumes a regular 28-day cycle with day-14 ovulation — irregular or unknown periods reduce accuracy
  2. Conception date: reliable for IVF (embryo transfer date known exactly) — add 266 days
  3. Ultrasound dating: crown-rump length in the first trimester is the clinical gold standard, accurate to ±3–5 days; later scans are ±1–2 weeks and should not re-date a pregnancy
  4. Post-term: passing 42 weeks prompts induction discussion — that is why the 42-week checkpoint appears in countdowns

🔗 Pregnancy Health Cross-References

A due date anchors every prenatal milestone — scans, glucose screening, and trimester transitions are all scheduled relative to the 40-week gestational clock. Explore our 💪 Health Hub for more body and wellness tools.

Pregnancy nutrition and hydration needs rise steadily — keep macros and water intake on target with the related tools below — and price the $2,200 federal 🍼 Child Tax Credit your newborn adds once the tax year closes.

💧Daily Water Intake🥗Macro Calculator⚖️BMI & Calorie Calculator⏳Days Until Countdown🍼$2,200 Child Tax Credit

Frequently Asked Questions (Due Date Calculator)

How is the pregnancy due date calculated?

By Naegele's rule: take the first day of the last menstrual period (LMP) and add 280 days — equivalently subtract 3 months, add 1 year, then add 7 days. With a cycle longer or shorter than 28 days, shift the result by (cycle length − 28) days.

How accurate is a due date?

For dating by first-trimester ultrasound, about ±3–5 days. When based on an assumed regular 28-day cycle from LMP alone, expect roughly ±2 weeks — which is why the 'due date' is treated as a window (37–42 weeks) rather than a fixed appointment.

Can I use my conception date instead of LMP?

Yes — add 266 days to the conception date. This equals LMP + 280 because gestation is counted from about two weeks before fertilization. IVF pregnancies use the transfer date for exact dating.

What if my cycles are irregular or I don't know my LMP?

Cycle-based estimates become unreliable with irregular periods (variance over ~7 days shifts the date accordingly). Use an early ultrasound for dating, and enter the LMP you do have — the calculator's cycle adjustment compensates for consistently long or short cycles (21–45 days).

Do twins come earlier than the due date?

Twin pregnancies typically deliver earlier — around 36–37 weeks on average — so the standard 40-week date is less predictive. Your provider will set a twin-specific timeline, usually with more frequent growth scans from the second trimester.

What do 'weeks pregnant' and gestational age mean?

Gestational age counts completed weeks since the first day of the last period, so it runs two weeks ahead of the embryo's actual age. At 14w 6d you are 14 completed weeks plus 6 days, in the second trimester, with 25 weeks (175 days) to the 40-week mark.