Pregnancy dating is one of the first questions every expectant parent asks, and it drives almost every appointment, ultrasound, and prenatal test on the calendar. This pregnancy calculator uses Naegele's rule — the same method taught in obstetrics training — to estimate your due date from the first day of your last menstrual period (LMP), then adjusts for your actual cycle length so women with longer or shorter cycles get a more realistic date than the rough "minus three months plus seven days" rule of thumb.
Because only about 5% of babies arrive on their exact estimated due date, think of the result as a window rather than a deadline. Roughly 80% of healthy first pregnancies deliver somewhere between 38 and 42 weeks, and an early ultrasound is the gold standard for refining the date. Still, the LMP estimate is the starting point your clinician will use, and it is good enough for planning maternity leave, scheduling prenatal visits, and wrapping your head around how far along you are.
How to Use
- Enter the Year, Month, and Day of the first day of your last menstrual period. This is day 1 of your cycle, not the last day of bleeding. If you are unsure, use the date you started spotting.
- Set your Average Cycle Length in days. The default is 28 days, which is the textbook assumption. If your cycles are regularly 30, 32, or 35 days, enter that number — the calculator shifts your due date later by one day for every day past 28.
- Read the result card: the Estimated Due Date is your headline number, followed by the estimated conception date and the dates you will finish the first and second trimesters.
- Use the trimester milestones to plan: the first trimester ends around week 12 (when the risk of miscarriage drops and the anatomy scan is booked), and the second ends around week 27 (when viability outside the womb becomes realistic).
- If you have had a first-trimester ultrasound, bring the ultrasound dating to your clinician. If it disagrees with the LMP estimate by more than about 5–7 days, the ultrasound usually wins.
The Due Date Formula
Naegele's rule is simple: take the first day of your last period, add 280 days (40 weeks), and that is your estimated due date. The classic shortcut is to subtract three months from the LMP date and add seven days — for example, January 1 becomes October 8.
That shortcut assumes a textbook 28-day cycle in which ovulation happens on day 14. If your cycle is longer, ovulation happens later, so the pregnancy clock should start later as well. The calculator applies a one-day adjustment for every day your cycle differs from 28:
- Estimated due date = LMP + 280 days + (cycle length − 28)
- Estimated conception = LMP + (cycle length ÷ 2)
So a woman with a 32-day cycle ovulates around day 16 rather than day 14, and her due date is pushed out by four days. Women with very irregular cycles should not over-rely on the LMP estimate; an early ultrasound is far more accurate.
Examples
Example 1 — A textbook 28-day cycle. An expectant mother's last period started on January 1, 2025, and her cycles are 28 days long. The calculator adds 280 days and gives an estimated due date of October 8, 2025, with an estimated conception around January 15, 2025, and the end of the first trimester on March 26, 2025.
Example 2 — Longer cycles shift the date. Her last period started June 15, 2025, but her cycles run 32 days. The default Naegele math would suggest March 22, 2026, but the four-day cycle correction pushes the real estimate out to March 26, 2026. That four-day shift can move her anatomy scan and her estimated c-section date by almost a week.
Example 3 — A leap-year LMP. A period that began on February 29, 2024 (a leap day) still adds 280 days cleanly to give a due date of December 5, 2024, with conception estimated around March 14, 2024.
Example 4 — Planning around the 40% marker. By the time you reach the start of the third trimester (around week 28), you have completed roughly 70% of a full-term pregnancy. Use the first- and second-trimester milestone dates on this page to pace your announcements, nursery build-out, and maternity-leave planning.
FAQ
How is my due date calculated?
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, then adjust by the difference between your actual cycle length and 28 days. This calculator does exactly that and also shows the estimated conception date, which falls about halfway through your cycle.
What if my cycles are irregular?
The LMP method assumes you ovulated on day 14 of a 28-day cycle. If you ovulated much later or earlier, the estimate can be off by a week or more. In that case, ask your clinician for a first-trimester ultrasound, which measures the embryo's crown-rump length and is the most accurate way to date a pregnancy in the first trimester.
How accurate is the due date?
Only about 5% of babies are born on their exact estimated due date. Around 80% of first-time mothers deliver within the window between 38 and 42 weeks, and 90% deliver within two weeks of the estimate. Treat the date as a planning anchor, not a finish line.
Why is pregnancy counted from my last period and not conception?
Obstetric dating starts on the first day of your LMP because that date is easy to remember, while the exact day of ovulation is usually unknown. This means that at the moment of conception you are already "about 2 weeks pregnant" in clinical terms. By the time you miss a period, you are roughly 4 weeks along.
When does each trimester end?
The first trimester ends around week 12 (about 84 days after your LMP), the second ends around week 27 (about 189 days), and the third runs from week 28 until delivery. This calculator shows you the calendar date each milestone falls on, so you can book scans and prenatal appointments around them.
Does my cycle length really change the due date?
Yes, by about one day per day of deviation from 28. A woman with regular 35-day cycles ovulates roughly a week later than the textbook assumption, and her due date should be shifted out by about seven days. If you enter your real cycle length here rather than the 28-day default, the estimate will match what your OB/GYN calculates.
Disclaimer
This pregnancy calculator is provided for informational purposes only and does not constitute medical, obstetric, or midwifery advice. Due-date estimates are based on Naegele's rule and your entered cycle length; actual pregnancy duration varies widely and may be revised by your clinician based on ultrasound findings, fertility treatments, or known ovulation dates. Always confirm your estimated due date, prenatal schedule, and any symptoms of concern with a qualified healthcare provider.