Pregnancy Due Date Calculator
Solution
Medical disclaimer: This is an estimate based on standard obstetric formulas. Only 4–5% of babies are born on their estimated due date. Always confirm with your healthcare provider.
Naegele's Rule (from LMP)
The standard obstetric formula: add 280 days (40 weeks) to the first day of the last menstrual period to estimate the due date for a typical 28-day cycle.
EDD = LMP + 280 days
From Conception Date
Fertilization happens about 14 days after the LMP in a 28-day cycle, so add 266 days (38 weeks) to a known conception date.
EDD = Conception + 266 days
From Ultrasound Gestational Age
If a sonographer measured the fetus and reported a gestational age of weeks + days on the ultrasound date, add the remaining days to reach 280 to estimate the due date.
EDD = Ultrasound date + (280 − gestational age in days)
How It Works
The pregnancy due date calculator applies Naegele's rule, the standard obstetric formula. Counting starts on the first day of the last menstrual period (LMP), and the estimated due date is 280 days (40 weeks) later. Conception is assumed to happen on day 14 of a 28-day cycle, so the same due date can be reached from a known conception date by adding 266 days, or from an ultrasound by counting forward from the gestational age the sonographer measured. Only about 4–5% of babies are born on their estimated due date — most arrive within two weeks before or after.
Example Problem
What is the estimated due date for a woman whose last menstrual period began on January 1, 2026?
- Identify the input: LMP = January 1, 2026.
- Apply Naegele's rule: EDD = LMP + 280 days.
- Count forward 280 days from January 1, 2026 — 30 days remain in January, plus 28 in February (2026 is not a leap year), 31 in March, 30 in April, 31 in May, 30 in June, 31 in July, 31 in August, 30 in September, totaling 272 days through September 30, 2026.
- Add the remaining 8 days into October to reach 280 days.
- Result: EDD = October 8, 2026.
- Implied conception date = LMP + 14 days = January 15, 2026, and on the EDD the pregnancy will be 40 weeks (third trimester).
ACOG (American College of Obstetricians and Gynecologists) treats Naegele's rule as the standard reference unless ultrasound dating (typically before 22 weeks) suggests a different EDD by more than the threshold for the gestational-age window.
Key Concepts
Naegele's rule assumes a regular 28-day menstrual cycle with ovulation on day 14. People with longer or shorter cycles ovulate earlier or later, so the LMP-based EDD can drift by several days. Gestational age (GA) — the number of completed weeks since LMP — is the obstetric convention; embryonic age (or fetal age, time since conception) is roughly two weeks less. A first-trimester ultrasound (before about 14 weeks) is considered the most accurate method for dating a pregnancy because the fetus grows at a predictable rate at that stage; later ultrasounds become progressively less accurate as growth varies more between individuals.
Applications
- Scheduling prenatal visits, glucose screening (24–28 weeks), and the anatomy ultrasound (18–22 weeks)
- Calculating maternity-leave start dates and planning for FMLA eligibility windows
- Timing the Tdap vaccine recommendation (27–36 weeks) and Group B Strep screening (36–37 weeks)
- Identifying preterm (< 37 weeks), early-term (37–38 weeks), full-term (39–40 weeks), late-term (41 weeks), and post-term (≥ 42 weeks) delivery windows
- Coordinating childcare, hospital pre-registration, and travel restrictions in late pregnancy
Common Mistakes
- Relying on Naegele's rule when the cycle is significantly longer or shorter than 28 days — adjust by adding or subtracting the difference, or use ultrasound dating instead
- Conflating gestational age (weeks since LMP) with embryonic age (weeks since conception) — they differ by about 14 days
- Confusing the first day of the LMP with the last day — Naegele's rule starts from the FIRST day of bleeding, not the day the period ended
- Treating the due date as a deadline — only 4–5% of babies are born on their EDD; full term is 39 weeks 0 days to 40 weeks 6 days
- Re-dating based on a third-trimester ultrasound — late ultrasounds have ±2–3 week error margins and should not override a confident first-trimester EDD
Frequently Asked Questions
When is my due date?
If you know the first day of your last menstrual period (LMP), your estimated due date is 280 days (40 weeks) later. For example, an LMP on January 1, 2026 gives a due date of October 8, 2026. Enter your LMP into the calculator above to see the exact date, along with your current gestational age and trimester.
How is the due date calculated?
The standard formula is Naegele's rule: EDD = LMP + 280 days. The 280 days assumes a regular 28-day cycle, with conception about 14 days after the LMP and an additional 266 days of pregnancy. Equivalently, you can subtract 3 months from your LMP and add 7 days and 1 year — that historical shortcut produces the same date.
What is Naegele's rule?
Naegele's rule is a 19th-century obstetric formula named after the German obstetrician Franz Karl Naegele. It estimates a 40-week (280-day) pregnancy starting from the first day of the LMP. It is the foundation of obstetric dating in the United States and is the formula the American College of Obstetricians and Gynecologists (ACOG) uses unless first-trimester ultrasound dating disagrees by more than the redating threshold.
Should I use my LMP or my conception date?
Use whichever you know more confidently. The LMP is more commonly remembered and is the obstetric standard. The conception date is more direct but is rarely known precisely unless conception happened through IVF or carefully tracked ovulation. The calculator above gives the same EDD from either input when the 14-day cycle offset holds.
Why is ultrasound dating more accurate after an irregular cycle?
If you have an irregular or non-28-day cycle, ovulation does not happen on day 14, so the 280-day count from LMP can be off by a week or more. A first-trimester ultrasound measures the fetus directly (crown-rump length) — at that stage all fetuses grow at nearly the same rate, so the measured gestational age is reliable to within ±5 to 7 days. After 22 weeks, individual growth variation makes ultrasound dating progressively less accurate.
What if my cycle isn't 28 days?
If your cycle is longer than 28 days, ovulation happens later in the cycle and the actual conception date is later than day 14 of your LMP. As a quick adjustment, add the extra days (e.g., for a 32-day cycle, add 4 days to the LMP-based EDD). For shorter cycles, subtract the difference. For more accuracy, ask your provider about confirming the EDD with a first-trimester ultrasound.
Is my due date exact?
No — only about 4–5% of babies are born on their estimated due date. The EDD is the midpoint of a normal full-term delivery window. A pregnancy is considered full term from 39 weeks 0 days to 40 weeks 6 days, early term from 37 to 38+6 weeks, and late term at 41 weeks. The calculator gives the midpoint estimate; your provider will use it to plan prenatal care, not to predict the exact day of birth.
How do I calculate gestational age (how far along am I)?
Gestational age is the number of completed weeks since the first day of your LMP. Subtract the LMP from today and convert to weeks + days. For example, if today is 100 days past LMP, you are 14 weeks 2 days along — in the second trimester. The calculator above reports this automatically for whatever "as of" date you choose.
Reference: Naegele's rule (Franz Karl Naegele, 1812). ACOG Committee Opinion No. 700, "Methods for Estimating the Due Date," reaffirmed 2022. This is an estimate based on standard obstetric formulas — always confirm with your healthcare provider.
Worked Examples
From LMP
What is the due date for an LMP on January 1, 2026?
A textbook Naegele's rule case: count 280 days forward from the first day of the last menstrual period.
- Mode = Last Menstrual Period
- LMP = January 1, 2026
- EDD = LMP + 280 days
- January 1 + 280 days = October 8, 2026
- Implied conception = January 15, 2026 (LMP + 14)
the estimated due date is October 8, 2026.
From Conception Date
What is the due date if conception happened on June 15, 2026?
Useful for IVF transfers or carefully tracked ovulation — count 266 days forward from the conception date.
- Mode = Conception Date
- Conception = June 15, 2026
- EDD = Conception + 266 days
- June 15 + 266 days = March 8, 2027
- Implied LMP = June 1, 2026 (Conception − 14)
the estimated due date is March 8, 2027.
From Ultrasound
What is the due date if an ultrasound on April 1, 2026 measures 10 weeks 2 days?
First-trimester ultrasound dating overrides LMP when the cycle is irregular — count forward from the measured gestational age.
- Mode = Ultrasound
- Ultrasound date = April 1, 2026
- Gestational age at ultrasound = 10 weeks 2 days (72 days)
- EDD = Ultrasound date + (280 − 72) days = + 208 days
- April 1 + 208 days = October 26, 2026
the estimated due date is October 26, 2026.
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