Pregnancy dating almost always starts from a date you already know: the first day of your last menstrual period (LMP). This LMP due date calculator applies Naegele's rule — the same method obstetricians have used for over a century — to turn that single calendar date into your estimated due date (EDD), your likely conception date, and the dates of the major prenatal milestones, from the 20-week anatomy scan to the 36-week group B strep screen.
Type in the first day of your last period and your usual cycle length, and every date below updates instantly. The calculator is built for people with regular 21–35 day cycles; if your cycles are very irregular or you conceived via fertility treatment, your ultrasound-based dating scan will be more accurate, and your OB will adjust the EDD accordingly.
How to Use
- Enter the First Day of Last Menstrual Period (LMP). This is the day bleeding began — not the day it ended, and not the day you think you ovulated. If your last period started on January 1, 2026, enter 2026-01-01.
- Set your Average Cycle Length. The textbook default is 28 days, but only about 1 in 5 people actually have a 28-day cycle. If your cycles typically run 30, 32, or 35 days, enter that number — the calculator shifts your due date forward by the extra days.
- Optionally adjust the Luteal Phase Length. The default is 14 days, which is the time from ovulation to the next period for most people. If you track ovulation with LH tests or basal temperature and know your own luteal phase runs shorter (around 10–12 days) or longer (15–16 days), type that in — it shifts the estimated conception date and EDD accordingly.
- Read the Estimated Due Date (EDD). With a January 1, 2026 LMP and a 28-day cycle, Naegele's rule gives October 8, 2026.
- Check the Likely Birth Window — your EDD minus and plus two weeks. Roughly 80% of people deliver within this 4-week window, and only about 5% actually give birth on the exact due date.
- Scroll to the milestone dates: the end of the first trimester, the 20-week anatomy ultrasound, and the 36-week GBS screen. Use them to book prenatal appointments and ultrasound slots early, because popular sonography clinics fill up fast.
The result card shows eight dates: the EDD, the estimated conception date, the ±2-week birth window, the first- and second-trimester boundaries, the anatomy scan date, and the GBS screening date.
The Naegele Rule Behind the Math
Naegele's rule assumes a "textbook" pregnancy lasts 280 days (40 weeks) from the first day of the last period. The quick mental version is: subtract three months from the LMP, add one year, and add seven days. For January 1, 2026 that gives January 1 minus three months = October 1, 2025, plus one year = October 1, 2026, plus seven days = October 8, 2026 — the same result as adding 280 days.
Because ovulation typically happens about 14 days before the next period (the luteal phase), not exactly at the midpoint of every cycle, the calculator generalizes Naegele's rule in two steps:
Estimated conception = LMP + (cycle length − luteal phase) days EDD = estimated conception + 266 days (average gestation)
With the textbook 14-day luteal phase this collapses to the familiar Naegele shortcut: conception = LMP + (cycle − 14), and EDD = LMP + (cycle − 14) + 266 = LMP + cycle + 252 days — which for a 28-day cycle equals LMP + 280 days.
So on a 35-day cycle with a 14-day luteal phase, your EDD shifts seven days later than the 28-day estimate (LMP + 287 days). On a 30-day cycle, it shifts two days later. If your own luteal phase runs shorter (say 12 days) or longer (say 16 days), type that into the Luteal Phase Length box — it shifts the estimated conception date by the difference, and the EDD moves with it.
Why the Due Date Is an Estimate, Not a Deadline
It is worth repeating: only about 5% of pregnant people deliver on their exact EDD. First babies are born on average a few days later than subsequent ones, and a normal pregnancy is considered full-term from 39 weeks through 40 weeks + 6 days. Your provider will re-date the pregnancy at the 8–12 week ultrasound using crown-rump length; if that scan disagrees with your LMP-based EDD by more than a few days, the ultrasound date usually wins, especially in first pregnancies.
Examples
Example 1 — The textbook case. Your last period started January 1, 2026 and your cycles are a regular 28 days. The calculator gives an EDD of October 8, 2026, an estimated conception date of January 15, 2026, a birth window running from September 24, 2026 to October 22, 2026, a 20-week anatomy scan around May 21, 2026, and a 36-week GBS screen around September 10, 2026.
Example 2 — Longer cycles shift the date. Same January 1 LMP, but your cycles usually run 35 days with a typical 14-day luteal phase. Conception is estimated at January 22, 2026 (LMP + 21 days), and the EDD shifts to October 15, 2026. The 20-week anatomy scan, anchored to LMP rather than to EDD, still lands around May 21, 2026. Ignoring this cycle-length adjustment is one of the most common reasons home due-date calculators are off by a week or more for people with longer cycles.
Example 3 — Short cycles. If your LMP is January 1, 2026 and your cycles run 25 days with a 14-day luteal phase, conception lands on January 12, 2026 (LMP + 11 days), and the EDD pulls three days earlier to October 5, 2026.
Example 4 — A shorter-than-average luteal phase. Same January 1 LMP and 28-day cycle, but your LH tests show a 12-day luteal phase. Conception shifts to January 17, 2026 (LMP + 16 days), and the EDD moves two days later to October 10, 2026. Most people can leave luteal phase at 14 unless they are actively tracking ovulation.
FAQ
How do I calculate my due date from my last period?
Start with the first day of your last menstrual period. With a textbook 28-day cycle and a 14-day luteal phase, add 280 days (40 weeks) — for example, an LMP of January 1, 2026 gives an EDD of October 8, 2026. If your cycle length or luteal phase differs, the calculator shifts the date by the difference.
What if my cycles are not 28 days?
Enter your actual average cycle length. The calculator treats conception as LMP + (cycle length − luteal phase) days and adds 266 days of average gestation. A 35-day cycle pushes the EDD about a week later; a 25-day cycle pulls it a few days earlier. If you track ovulation with LH tests, you can also enter your own luteal phase length rather than leaving it at the 14-day default.
How accurate is Naegele's rule?
It is a useful first estimate, but only about 5% of people deliver on the exact EDD. Your OB will confirm or adjust the date at the 8–12 week ultrasound using crown-rump length; that first-trimester scan is more accurate than LMP alone, especially when cycles are irregular.
What is the difference between LMP dating and conception dating?
LMP dating counts 40 weeks from the first day of your last period, which includes roughly two weeks before you actually ovulated. Conception dating counts 38 weeks from the estimated conception date. They describe the same pregnancy; the medical community uses LMP dating because the date is easy to recall and the two-week offset is built into the 280-day formula.
When is my third trimester?
The first trimester ends at 13 weeks + 0 days (about LMP + 91 days). The second trimester runs from 13 weeks + 1 day through 27 weeks + 6 days. The third trimester starts at 28 weeks (about LMP + 196 days) and continues until birth. This calculator shows the first- and second-trimester boundaries on the result card.
What prenatal appointments should I book around my due date?
Use the milestone dates to schedule: the 8–12 week dating scan, the 20-week anatomy ultrasound, the 24–28 week glucose screening, the 28-week Rhogam shot for Rh-negative patients, and the 36-week group B strep (GBS) culture. Popular clinics book out several weeks ahead, so lock these in as soon as your EDD is confirmed.
Disclaimer
This LMP due date calculator is provided for informational purposes only and is not medical advice. Due dates, milestone dates, and conception estimates are approximations based on Naegele's rule and an assumed 280-day pregnancy. Actual gestational age and delivery timing vary widely, and ultrasound-based dating, your medical history, and guidance from your obstetrician or midwife always take precedence. Never use this calculator to make medical decisions, stop or start treatment, or time a procedure without speaking to a qualified healthcare professional.