Last Menstrual Period (LMP) Due Date Guide

What Is the LMP Method?

The last menstrual period (LMP) method is the oldest and still most universal way to date a pregnancy. It needs no equipment, no appointment, and no special knowledge — just the date your last period began. From that single date, providers derive gestational age, schedule screening windows, and decide when monitoring should start.

Its staying power comes from practicality. The LMP is usually knowable weeks before a pregnancy is even confirmed, so it provides a working timeline from the very first phone call to a provider’s office. Everything that follows — blood screening timing, the anatomy scan window, when to watch for post-term — is first anchored to this date and only revised if later evidence demands it.

The History of Naegele’s Rule

The rule is named for Franz Karl Naegele (1778–1851), a German obstetrician and professor at Heidelberg who described it in the early 19th century. His formulation was disarmingly simple: add one year to the first day of the last period, subtract three months, then add seven days. The result — 280 days forward — became the definition of a 40-week pregnancy.

What is remarkable is how little the core idea has changed in two centuries. Naegele worked without ultrasound, without hormone tests, and without any reliable way to detect ovulation; the last period was the only fixed biological landmark available. The rule was later refined with the cycle-length correction (the so-called modified Naegele’s rule), which adjusts for the fact that ovulation does not always fall on cycle day 14. Modern practice layered ultrasound confirmation on top — but the LMP date remains the first thing every provider asks about.

Formula Variants: 280 Days vs 266 Days

Several formulations describe the same arithmetic, and mixing them up is a common source of confusion. The 280-day count starts at the first day of the last period and measures to the due date — this is gestational age, the count every provider uses. The 266-day count starts at conception (or ovulation) and measures to the same due date — this is embryonic or fetal age. Because conception typically falls about 14 days after the LMP, 280 minus 14 equals 266: both roads reach the same destination.

Naegele’s original recipe — add a year, take back three months, add seven days — is algebraically identical to adding 280 days. Some older textbooks also taught a nine-months-plus-seven-days shortcut, which approximates the same span but can drift by a few days depending on month lengths; the 280-day version is the precise one worth using. For non-28-day cycles, every variant gets the same correction: add (cycle length − 28) days to the result.

Irregular, Long, and Short Cycles

The standard 280-day formula quietly assumes you ovulated on cycle day 14. But ovulation actually tends to occur about 14 days before your next period begins — so the day of ovulation depends on your total cycle length. In a 35-day cycle, ovulation lands around day 21; in a 21-day cycle, around day 7. Each day of difference shifts your due date by a day.

Long cycles (30–45 days)

Longer cycles push ovulation — and therefore conception — later than the textbook assumption. A 35-day cycle adds 7 days to the standard result; a 40-day cycle adds 12. This correction matters most when cycles are consistently long, because the unadjusted formula would date the pregnancy a full week or more younger than it really is, which can shift screening windows and cause unnecessary worry about slow early growth.

Short cycles (21–27 days)

Short cycles pull ovulation earlier. A 21-day cycle subtracts 7 days from the standard result. Without the correction, the formula overestimates gestational age, which can make a pregnancy look further along than it is — a common reason early scans surprise people by “measuring behind.”

Irregular cycles

When cycles swing between, say, 24 and 40 days, no single cycle-length number captures what happened in the conception month. Use your average cycle length as the input and treat the result as approximate — a wider estimate rather than a precise date. If you tracked ovulation that month with a predictor kit or temperature charting, dating from ovulation is the far more reliable option for that pregnancy.

Worked Examples

Example 1 — textbook 28-day cycle. LMP: January 1, 2026. No adjustment needed. January 1, 2026 + 280 days = October 8, 2026.

Example 2 — 35-day cycle. LMP: January 1, 2026. Correction: +7 days. January 1, 2026 + 280 days + 7 days = October 15, 2026.

Example 3 — 21-day cycle. LMP: January 1, 2026. Correction: −7 days. January 1, 2026 + 280 days − 7 days = October 1, 2026.

Example 4 — 30-day cycle. LMP: March 1, 2026. Correction: +2 days. March 1, 2026 + 280 days + 2 days = December 8, 2026. Notice how even a small two-day cycle difference moves the due date — precision here costs nothing and can prevent a misdated early scan.

When LMP Dating Fails

The LMP method is only as good as its two inputs: the remembered date and the ovulation assumption. A genuinely forgotten LMP — or one remembered only as “sometime in March” — leaves too much uncertainty for screening schedules, and the honest move is to date by early ultrasound instead. Spotting can also masquerade as a period: light bleeding in early pregnancy is sometimes logged as an LMP, which can misdate the pregnancy by weeks.

The ovulation assumption breaks down after recent hormonal birth control, while breastfeeding, or during the postpartum return of fertility, when ovulation timing can be wildly atypical. Very irregular cycles, polycystic ovary syndrome, and perimenopause all make day-14-style assumptions unreliable. None of these situations mean anything is wrong — they simply mean the LMP date deserves a confirming scan rather than blind trust.

LMP vs Ultrasound Dating

Modern practice does not choose between LMP and ultrasound — it uses both in sequence. The LMP date provides the initial working estimate from the first contact; a dating ultrasound, ideally before 14 weeks, then confirms or corrects it. Crown-rump length measurement in early pregnancy is the most precise dating tool available, with an uncertainty of roughly five to seven days in the first trimester.

The general rule providers follow: if the first-trimester ultrasound differs from the LMP-based date by more than about 7 days (or more than about 10 days for a second-trimester scan), the ultrasound measurement becomes the official estimated due date. A small correction is routine and does not reflect on the pregnancy’s health — it simply means the calendar is being calibrated to the more precise instrument. If you have scan measurements, you can run the numbers yourself with our ultrasound due date calculator, and see what each milestone looks like in our week-by-week pregnancy guide.

How Providers Use Your LMP Date

At your first prenatal visit, the LMP date you provide becomes the seed for your entire care schedule. Your provider will ask for the first day of your last period, your typical cycle length, and whether you tracked ovulation or used fertility treatment — then combine those answers with the dating ultrasound to lock in your official estimated due date.

What the EDD schedules

Nearly every time-sensitive decision in prenatal care hangs off the EDD. First-trimester screening windows (roughly weeks 11–14), the anatomy scan (roughly weeks 18–22), glucose screening (roughly weeks 24–28), and the point at which a pregnancy is considered post-term all count forward from the dating you established at the start. A due date that is off by a week can shift all of these — which is exactly why providers confirm the LMP date with an early scan rather than taking it on faith.

How corrections get recorded

When the dating scan disagrees with the LMP date beyond the usual threshold, your provider updates your records to the ultrasound-based date and uses it consistently from then on. Your original LMP stays in the chart as history, but all future scheduling follows the corrected date. If you ever transfer care or visit a new provider, mention both dates and which one your current provider treats as official — it prevents the two calendars from quietly diverging.

Getting the Most from Your LMP Date

For this pregnancy: use the first day of full bleeding, apply your cycle-length correction, and bring your workings to your first appointment — providers genuinely appreciate seeing how you arrived at your date. If you are unsure of your LMP, say so plainly rather than guessing; an honest “I don’t remember” leads to a dating scan, while a confident wrong guess can misdirect early care.

For future pregnancies, the best preparation is a few months of simple record-keeping: the first day of each period and your average cycle length. That small habit turns the 200-year-old rule into a genuinely personal estimate — and gives your future provider exactly what they need on day one.

A final note on due date anxiety

It is easy to treat the EDD as a deadline, especially when apps count down the days. Remember what the word estimated is doing: your due date is the midpoint of a five-week window of normal, healthy birth timing. Babies who arrive at 38 weeks or 41 weeks are not “early” or “late” in any meaningful sense — they are simply landing at different points in the window the method was always describing. Use the date for planning and appointments, and let the window do the rest.