A Little Math for a Big Milestone
One of the first questions every newly expectant parent asks — usually within minutes of the positive test — is when? Doctors answer it with a method that has been used for over two centuries: count 40 weeks from the first day of your last menstrual period. It feels odd, since conception happens about two weeks later, but it works because the period’s first day is a date most people can actually remember.
This free pregnancy due date calculator does that arithmetic for you in one click. Enter the first day of your last period and you’ll get your estimated due date, how far along you are today (weeks and days), your trimester, your likely conception date, and a gentle progress view of the journey so far.
Naegele’s rule · LMP + 280 days
Please enter the first day of your last menstrual period (a date in the past).
Your estimated due date
How to Use This Calculator
- Enter the first day of your last menstrual period. That is day 1 of bleeding — the day your period started, not the day it ended. Pick it from the calendar.
- Click Calculate Due Date. Your estimated due date appears at the top, with your current week and day, trimester, days remaining and likely conception date below.
- Read the results gently. This is an estimate based on a regular 28-day cycle. Your doctor or midwife will confirm the date — often adjusting it after an early ultrasound.
What Is Naegele’s Rule?
Naegele’s rule — named for the 19th-century German obstetrician Franz Karl Naegele — is the standard way due dates have been estimated for generations. It assumes a regular 28-day menstrual cycle with ovulation (and therefore conception) around day 14, and a pregnancy lasting 280 days, or 40 weeks, from the first day of the last period.
The classic shortcut goes: subtract three months from the first day of your last period, then add one year and seven days. Last period started January 1, 2026? Minus three months is October 1, 2025; plus a year and seven days is October 8, 2026. The calculator simply adds 280 days — the same result, without the calendar gymnastics.
It is worth knowing its limits: the rule assumes regular 28-day cycles, and real cycles vary. An early ultrasound (measuring the baby in the first trimester) is considered the most accurate dating method, and if it disagrees with the period-based date by more than about a week, most providers go with the ultrasound date.
The Formula (With a Worked Example)
Likely conception date ≈ first day of last period + 14 days (assumes a 28-day cycle)
Worked example. First day of last period: January 1, 2026.
- Estimated due date: January 1, 2026 + 280 days = October 8, 2026
- Likely conception: January 1 + 14 days = January 15, 2026
- On October 11, 2026, that is 283 days along — 40 weeks and 3 days, in the third trimester, with the estimated date just passed
Another quick check: a last period starting March 15, 2026 gives an estimated due date of December 20, 2026, with likely conception around March 29.
Tips for the Weeks Ahead
- Book your first prenatal visit early. Most providers like to see you around 8–10 weeks. Bring the first day of your last period — they will ask.
- Hold the date lightly. Only about 1 in 20 babies arrives on the estimated due date. The full-term window runs roughly from week 37 to week 42, and anywhere in there is normal.
- Know your trimesters. First: weeks 1–13, second: 14–27, third: 28–40. Each brings its own milestones, screenings and symptoms to discuss at appointments.
- Irregular cycles? Say so. If your cycles run much shorter or longer than 28 days, mention it at your first visit — your provider will likely date the pregnancy by early ultrasound instead.
- Start the healthy habits now. A prenatal vitamin with folic acid, no alcohol or smoking, and regular gentle movement are the standard early recommendations — your provider will personalize them.
- Keep every appointment. Especially if you go past your estimated date: providers monitor closely after 40 weeks and usually discuss next steps around 41–42 weeks.
Related Tools
What Happens When: Trimester Milestones
Pregnancy is measured in three trimesters, and a few dates inside them are worth circling:
- First trimester (weeks 1–12). The embryo develops its major organs; a heartbeat is usually detectable by ultrasound around weeks 6–8.
- Second trimester (weeks 13–26). Most people start feeling movement around weeks 18–22, when the detailed anatomy scan also happens.
- Third trimester (weeks 27–40). The baby gains weight quickly, the lungs mature, and provider visits become more frequent.
One fact worth knowing early: only about 5% of babies are born on their exact due date. The date is the center of a window, not an appointment. Providers describe the end of pregnancy in stages — early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks), and post-term (42 weeks or beyond).
FAQs
How accurate is a due date calculated from my last period?
It is a good estimate for regular 28-day cycles, usually within a week or two of the true date. Accuracy drops with irregular cycles, and only about 5% of babies arrive exactly on the estimated day — most come within two weeks either side. An early ultrasound is the most precise dating method.
Why is pregnancy counted from my last period instead of conception?
Because the first day of bleeding is a memorable, observable date, while the exact day of conception usually isn’t. Counting from the period adds roughly two weeks of “pregnancy” before conception — which is why a “40-week” pregnancy is really about 38 weeks from conception.
My cycles are irregular. Will this still work?
Less reliably. Naegele’s rule assumes ovulation on day 14 of a 28-day cycle; longer or shorter cycles shift conception earlier or later, moving the true due date. Tell your provider about your cycle pattern — they will very likely date the pregnancy with an early ultrasound instead.
How is the likely conception date estimated?
By adding 14 days to the first day of your last period — the assumed ovulation day in a textbook 28-day cycle. It is a rough guide, not a fact: ovulation can happen days earlier or later, and sperm can survive several days, so the real conception date may differ.
What are the trimesters, exactly?
The first trimester runs weeks 1–13, the second weeks 14–27, and the third weeks 28–40. Boundaries vary slightly between sources, but this 14/28-week split is the most widely used.
What if my due date passes and nothing happens?
Try not to worry — going past the estimated date is common, especially with a first baby. Your provider will monitor you more closely after 40 weeks (often with extra checks) and will usually discuss options such as induction if pregnancy reaches 41–42 weeks. Keep every appointment and report reduced movement immediately.
Can I use an IVF transfer date or ultrasound date instead?
Those are actually more precise than a period-based estimate. With IVF, providers date from the embryo transfer (adjusted for embryo age); with ultrasound, from first-trimester measurements. This calculator is designed for the common case — dating from the last period — so use your provider’s date when you have one.
How many babies are actually born on their due date?
Only about 5%. Most babies arrive within a week or two on either side of the due date, which is why providers talk about early term (37–38 weeks), full term (39–40 weeks), and late term (41 weeks) instead of treating one day as a deadline.
What counts as a full term pregnancy?
Full term is 39 to 40 weeks. Babies born at 37–38 weeks are called early term, those at 41 weeks are late term, and 42 weeks or beyond is post-term. Due date calculators target 40 weeks, which is why a healthy birth a little before or after the estimate is completely normal.
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