Natural Pregnancy Due Date Calculator
Estimate your due date from last menstrual period (LMP), suitable for natural conception and IUI
Due date = last period + 280 days (40 weeks)
How does this calculator work?
- 1 Enter the first day of your last menstrual period
- 2 Get your estimated due date, current gestational age, and days remaining automatically
How Is a Natural Pregnancy Due Date Calculated?
A natural due date is the first day of your last period + 280 days (40 weeks). Based on Naegele’s rule, this is the obstetric standard for women with regular ~28-day cycles, and per ACOG it is the method used worldwide.
Actual delivery can fall within 2 weeks either side of the due date; only about 5% happen on the day. Most births occur between weeks 37 and 42, and providers usually consider induction after week 41 because post-term pregnancy (beyond 42 weeks) raises fetal risk.
With irregular cycles, an early ultrasound (weeks 8–12) should correct the date. Ultrasound measurement of fetal crown-rump length (CRL) estimates gestational age more accurately. ACOG Practice Bulletin #229 recommends following the ultrasound when it differs from LMP dating by more than 5 days.
Compared with IVF dating, natural conception dating has a wider error margin (±7–14 days) because ovulation is estimated. IVF dates carry only ±1–2 days of error since fertilization is precisely known. If your cycles are irregular, combine ovulation records, intercourse dates, and early ultrasound results.
Naegele’s rule works like this: subtract 3 months (or add 9) from the LMP month and add 7 days. For example, an LMP of January 15 gives a due date of October 22. Described by German obstetrician Franz Carl Naegele in the 19th century, it remains the global standard.
What Is the Due Date Formula?
Based on Naegele’s rule: subtract 3 months (or add 9) from the first day of your last period and add 7 days — equivalent to counting 280 days (40 weeks) from the first day of your last menstrual period.
LMP + 280 days = due date What Are the Precautions for Due Date Calculation?
- • This method works best for regular cycles of about 28 days
- • If your cycles are irregular, rely on your early ultrasound result
- • A due date is an estimate — only about 5% of deliveries happen on the exact date
Frequently Asked Questions
What if my periods are irregular?
What if I go past my due date?
How accurate is a due date?
How can I date my pregnancy more accurately?
References
- [1] AAFP confirms Naegele’s rule — last menstrual period plus 280 days — as the most common method for estimating a due date — AAFP - Determining the Estimated Date of Confinement
- [2] ACOG recommends correcting the due date by first-trimester ultrasound, especially with irregular cycles; if ultrasound and LMP dating differ by more than 5 days, the ultrasound result prevails — ACOG Practice Bulletin #229
- [3] Mayo Clinic notes that only about 5% of pregnant people deliver on their exact due date; most deliver between weeks 37 and 42 — Mayo Clinic - Pregnancy Due Dates
- [4] The WHO divides pregnancy into three trimesters: first (weeks 1–12), second (weeks 13–27), and third (weeks 28–40) — WHO - Pregnancy Trimesters
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