Due Date Calculator
Estimate your expected delivery date from the first day of your last menstrual period and see how far along you are today.
How to use this due date calculator
- Recall your last period
Identify the first day of your most recent menstrual period.
- Enter the date
Select that date in the last menstrual period field.
- Review due date
Check the estimated due date calculated by adding 280 days to your LMP.
- Check gestational progress
Review your current pregnancy week and the number of days remaining.
- Discuss with your provider
Share the estimate with your healthcare provider, who may refine it with ultrasound data.
How this due date calculator works
This due date calculator estimates the expected delivery date by applying Naegele's rule to the first day of the last menstrual period (LMP). Naegele's rule has been the clinical standard for pregnancy dating since the early 19th century and remains the initial method used in most prenatal care settings worldwide. The calculator also shows the current gestational week and days remaining, giving you a practical timeline to discuss with your healthcare provider.
Estimated due date (EDD) = LMP + 280 days If the first day of the last menstrual period was January 1, adding 280 days gives an estimated due date of October 8. At 12 weeks past the LMP, the calculator would show the pregnancy as roughly in week 12 of 40 with approximately 196 days remaining.
If the first day of the last menstrual period was March 15, adding 280 days gives an estimated due date of December 20. At 20 weeks past the LMP, the calculator would show the pregnancy as roughly in week 20 of 40 with approximately 140 days remaining.
If the first day of the last menstrual period was August 10, adding 280 days gives an estimated due date of May 17. At 8 weeks past the LMP, the calculator would show the pregnancy as roughly in week 8 of 40 with approximately 224 days remaining.
- ✓ The calculation assumes a regular 28-day menstrual cycle with ovulation occurring around day 14.
- ✓ Women with longer or shorter cycles may have an actual due date that differs from this estimate by several days.
- ✓ Naegele's rule dates from the LMP, not from conception — actual embryonic age is roughly two weeks less than the gestational age shown.
- ✓ First-trimester ultrasound dating is considered more accurate than LMP-based dating when cycle length is irregular or uncertain.
- Only about 4–5% of births occur on the exact estimated due date; most healthy deliveries happen within a two-week window around it.
- Your healthcare provider may adjust the due date based on first-trimester ultrasound measurements, especially if cycle length is uncertain.
- This calculator is for informational planning only and does not replace prenatal medical care.
- Naegele, F.C., 'Lehrbuch der Geburtshülfe,' 1830 — original rule description
- American College of Obstetricians and Gynecologists (ACOG), Committee Opinion No. 700: 'Methods for Estimating the Due Date,' 2017
- Mongelli, M. & Gardosi, J., 'Estimating the Date of Confinement: Ultrasonographic Biometry versus Certain Menstrual Dates,' American Journal of Obstetrics and Gynecology, 1996
What is gestational age?
Gestational age measures how far along a pregnancy is, counted from the first day of the last menstrual period rather than from the date of conception. This convention means that on the day of conception — typically around day 14 of a 28-day cycle — the pregnancy is already considered two weeks along. The full term of pregnancy is 40 weeks, or 280 days, under this system. Gestational age is the standard used in prenatal care worldwide because the LMP date is usually easier to recall with certainty than the exact date of ovulation or fertilisation. Healthcare providers use gestational age to schedule screenings, track fetal development milestones, and determine whether growth is progressing as expected. When a first-trimester ultrasound measurement differs from the LMP-based gestational age by more than about seven days, clinicians may revise the estimated due date to match the ultrasound.
Why due dates are estimates, not guarantees
Only about 4 to 5 percent of babies are born on their exact estimated due date. The majority of full-term deliveries occur within a window of 37 to 42 weeks, and a birth anywhere in that range is generally considered normal. Several factors contribute to this variability. Cycle length differs from person to person, so the assumed 14-day luteal phase may not apply. First pregnancies tend to go slightly longer on average, while subsequent pregnancies may be shorter. Maternal age, genetics, and overall health also play a role. For these reasons, the estimated due date is best treated as the centre of a probable delivery window rather than a fixed appointment. Preparing for a range of dates — having essentials ready by 37 weeks, for example — is a practical approach that many expectant parents find less stressful than fixating on a single calendar date.
Due date calculator FAQs
How accurate is a due date calculated from the last period?
It is a reasonable estimate for women with regular 28-day cycles, but only about 4–5% of babies arrive on the exact date. Most deliveries fall within a window of 37 to 42 weeks.
What if I do not know the exact date of my last period?
If the LMP date is uncertain, a first-trimester ultrasound is the most reliable way to estimate gestational age and due date.
Does cycle length affect the estimate?
Yes. Naegele's rule assumes a 28-day cycle. If your cycle is consistently longer or shorter, ovulation timing shifts and the due date may differ by several days.
Why do doctors sometimes change the due date after an ultrasound?
First-trimester ultrasound measures the embryo directly, which can be more accurate than LMP-based dating — especially when cycle length is irregular or the LMP date is uncertain.
Is this calculator a substitute for prenatal care?
No. It is a planning tool for personal reference. All pregnancy dating and medical decisions should involve your healthcare provider.