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Pregnancy Due Date and Week Calculator
Enter either your last menstrual period (LMP) or conception date to estimate your due date and current pregnancy week. The calculator uses standard obstetric conventions: 280 days from LMP (Naegele’s rule) or 266 days from conception, with adjustments for cycle length when provided.
Enter a date and click Calculate to see your estimated due date and current pregnancy week.
The estimated due date (EDD) is a clinical estimate, not a guarantee. Most deliveries occur within two weeks of the EDD. The “current week” reflects completed weeks of gestation at today’s date. For medical decisions, confirm dating with an ultrasound scan, especially if cycles are irregular or conception date is uncertain.
How the estimation works
Obstetric dating counts pregnancy from the first day of the last menstrual period, not from conception. Naegele’s rule adds 280 days (40 weeks) to LMP, assuming a 28-day cycle with ovulation on day 14. If your cycle differs, the calculator adjusts the EDD by the difference between your cycle length and 28 days. When using conception date, it adds 266 days (38 weeks).
Key conventions
Pregnancy duration is measured in completed weeks. A pregnancy at 12 weeks and 3 days is in the 12th week until 13 full weeks are reached. The due date is estimated at 40 weeks from LMP, which corresponds to 38 weeks from conception under standard assumptions.
Assumptions: Regular menstrual cycles, ovulation occurring 14 days before the next period, and a 40-week gestation from LMP. Covers: Singleton pregnancies dated from LMP or conception. Excludes: Irregular cycles, fertility treatments with known ovulation dates, multiple pregnancies, or cases where ultrasound dating supersedes LMP. Do not use if you have had assisted reproductive technology (ART) with exact embryo transfer dates; use your clinic’s dating instead.
Worked examples
LMP on January 15, 2026 with a 28-day cycle.
Estimated due date: calculating…
Current pregnancy week on today’s date: calculating…
LMP on March 1, 2026 with a 32-day cycle.
Estimated due date: calculating…
Current pregnancy week on today’s date: calculating…
Conception date of February 10, 2026.
Estimated due date: calculating…
Current pregnancy week on today’s date: calculating…
Common pitfalls when dating a pregnancy
Assuming conception date is known precisely
Sperm can survive up to 5 days in the reproductive tract, and the ovum survives about 24 hours, creating a fertile window of ~6 days. Unless you used ovulation tracking with confirmed ovulation, the conception date is an estimate. LMP-based dating is therefore more reliable for most people.
Ignoring cycle length variations
A 28-day cycle assumption can be off by a week or more if your cycle is consistently longer or shorter. Always enter your average cycle length when using LMP. If cycles vary by more than 3–4 days, clinical dating may rely on early ultrasound instead.
Using first positive pregnancy test date as a reference
Home pregnancy tests detect hCG, which appears in urine ~12–14 days after conception. This date is too late to serve as a reliable pregnancy start reference. LMP or known conception/ovulation dates are required for accurate dating.
Unusual but valid scenarios
Very short or long menstrual cycles
Cycles between 20–45 days are accepted by the calculator, but clinical practice may treat cycles outside 21–35 days as irregular. For cycles <20 or >45 days, consult a healthcare provider, as ovulation timing may not follow typical patterns.
Pregnancy achieved via fertility treatments
If you used ovulation induction or IUI, the calculator can still work if you enter the cycle start date as LMP and your average cycle length. For IVF with known embryo transfer date, use the conception method and enter the transfer date as the conception date (adjusting for embryo age if known).
Next steps after estimating your due date
| Action | When to do it | Why it matters |
|---|---|---|
| Schedule first prenatal visit | By 8–10 weeks gestation | Early ultrasound can confirm viability and refine due date if LMP is uncertain. |
| Start prenatal vitamins | As soon as pregnancy is confirmed | Folic acid in early pregnancy reduces neural tube defect risk. |
| Review medications and supplements | Immediately | Some medications are unsafe in pregnancy; consult your provider. |
| Confirm dating with ultrasound | 11–14 weeks (dating scan) | Most accurate for EDD if LMP is uncertain or cycles are irregular. |
Frequently asked questions
Why is my due date 2 weeks later than I expected based on conception?
Pregnancy is clinically dated from the first day of your last period, not from conception. This means at conception you are already considered ~2 weeks pregnant. The 40-week countdown starts at LMP, so the math adds 280 days to LMP (or 266 days to conception) to reach the same estimated due date.
Can I use this calculator if I had irregular periods before pregnancy?
You can enter your average cycle length, but the estimate may be less accurate. If your cycles varied significantly, or if you’re unsure of your LMP date, an early dating ultrasound (typically at 11–14 weeks) will provide a more reliable due date.
Does the calculator account for leap years?
Yes. The JavaScript Date object handles leap years automatically, so a due date calculation crossing February 29 will be correct for the applicable year.
Why does my healthcare provider give a different due date?
Providers may adjust the EDD based on early ultrasound measurements, which are considered more accurate than LMP dating in many cases. Ultrasound in the first trimester can date a pregnancy within ~3–5 days, while LMP dating has a margin of error of ~±2 weeks.
Sources and verification
- ACOG: Methods for Estimating the Due Date — Clinical guidance on LMP vs ultrasound dating and Naegele’s rule.
- March of Dimes: Due Date Calculator — Consumer-facing explanation of pregnancy dating conventions.