Due Date by LMP — Calculate from Your Last Period

Why Due Dates Are Always Estimates

A due date is a population-level reference point, not a scheduled arrival. Only about 5% of birthing people deliver spontaneously on their calculated due date, and roughly 80% deliver within a ±2-week window around it. Variation comes from normal biological differences in implantation timing, menstrual-cycle length, fetal growth patterns, and individual factors we do not fully measure with simple calculators. Your prenatal care team will adjust your official dating based on first-trimester ultrasound crown-rump measurement when needed, which is the most accurate reference window we have.

Method 1 — Naegele Rule (LMP + 280 Days)

Method 2 — Conception Date

If you know the approximate date of ovulation, intercourse, or IUI or IVF fertilization, a conception-date estimate adds 266 days (38 weeks, the average duration of human gestation from fertilization to term) instead of 280. This removes the two-week LMP-to-conception offset, so the raw calculation does not depend on cycle length. Keep in mind that sperm can survive inside the female reproductive tract for several days, so if you are estimating from intercourse the actual fertilization window can span multiple days.

Method 3 — IVF Transfer

Cycle Length and LMP Dating

Naegele rule assumes a 28-day cycle. For people with consistently longer or shorter cycles, a simple linear adjustment (shifting the estimate by [cycle length − 28] days) can be applied. However, because luteal-phase length is relatively stable across cycle lengths (about 14 days) while follicular phase varies, adjusting LMP-based estimates purely for cycle length is an approximation at best. For irregular cycles, conception-date or first-trimester-ultrasound dating is generally more reliable.

Trimester Split

Three standard trimesters are used for reference and prenatal-care scheduling. The first trimester runs from LMP through week 12 + 6 days and covers the period of highest sensitivity for embryological development. The second trimester spans week 13 through week 27 + 6 days and is the typical window for anatomical screening, prenatal sugar screening, and fetal-growth surveys. The third trimester runs from week 28 through delivery, with the viability reference point commonly placed at 24 weeks and the term reference window at 37–42 weeks.

Why LMP Is the Standard Reference for Pregnancy Dating

The first day of the last menstrual period (LMP) is the most widely used starting point for pregnancy dating because it is observable, recalled without equipment, and predates conception by roughly two weeks. The Naegele rule, named after the 19th-century German obstetrician Franz Karl Naegele, builds on this reference by adding 280 days to the LMP to estimate the due date. The 280-day figure reflects the average human gestation from LMP to delivery across a large population, assuming a 28-day cycle with ovulation on day 14. This method remains the default for initial pregnancy dating in most prenatal records and is the formula used by the calculator on this page.

Naegele's Rule — The 280-Day Formula

The Naegele formula is arithmetic. Take the LMP date, add 280 days (40 weeks), and the result is the estimated due date. Equivalently, add one year, subtract three months, and add seven days. The two expressions produce the same date. The 280-day total is split into 14 days from LMP to estimated ovulation plus 266 days from fertilization to term. Because the formula bakes in the 28-day-cycle / day-14-ovulation assumption, it works best for individuals whose cycles match that pattern. For cycles that are consistently longer or shorter, a cycle length adjustment improves the estimate.

How to Determine Your LMP Accurately

LMP is the first day of full-flow menstrual bleeding, not the first day of spotting. Record the calendar date in your tracking app or on paper as soon as a new period begins, because recall accuracy drops quickly over time. If you track cycles, your records will give a reliable LMP. If you do not track cycles, the LMP you recall may be approximate, which introduces dating uncertainty. Some individuals bleed lightly around the time of implantation (6 to 12 days after fertilization) and may mistake that spotting for a period — this is one common source of LMP error. When LMP is uncertain, an early ultrasound provides a more reliable dating reference.

Cycle Length Adjustment

The follicular phase (the days from LMP to ovulation) is the variable portion of the cycle, while the luteal phase (the days from ovulation to the next period) is usually stable at 12 to 16 days. If your cycles are consistently 35 days rather than 28, ovulation likely happens around day 21 rather than day 14 — a 7-day shift. The cycle length adjustment captures this shift: for each day your typical cycle exceeds 28 days, add one day to the Naegele estimate; for each day below 28, subtract one day. Use the average of your last 3 to 6 cycles as the entered cycle length for the most stable reference. Avoid using a single recent cycle, since cycle-to-cycle variation of a few days is normal.

LMP Method Accuracy vs Ultrasound

The LMP-based estimate is accurate within roughly ±2 weeks for the population as a whole, because cycle lengths, ovulation timing, and gestation length all vary between individuals. Only about 5 percent of pregnancies deliver spontaneously on the calculated due date, with roughly 80 percent delivering within a ±2 week window around it. A first-trimester ultrasound (8 to 12 weeks) measuring the crown-rump length (CRL) is the most accurate dating reference in early pregnancy, with a margin of about 5 to 7 days. If the ultrasound-based dating differs from the LMP-based dating by more than 5 to 7 days in the first trimester, the care team will typically use the ultrasound-adjusted dating for all subsequent scheduling. The LMP method is a strong starting reference; the first-trimester ultrasound refines it.

When LMP Dating Is Less Reliable

LMP-based dating is less reliable when cycles are irregular, when LMP recall is uncertain, when recent hormonal contraception use has altered bleed patterns, when implantation bleeding was mistaken for a period, or when breastfeeding suppresses normal cycles. In these situations, an early ultrasound is the preferred dating reference. The calculator remains useful as an initial estimate, but the care team will rely on the ultrasound for the gestational age used in scheduling prenatal visits and milestone checks.

Practical Reference Tips

• Record the first day of full-flow bleed as day 1 of your cycle. Avoid counting spotting as the LMP.
• Enter the average of your last 3 to 6 cycle lengths for the adjustment, not a single recent cycle. Cycle-to-cycle variation of a few days is normal.
• Use the Naegele estimate as the initial reference. Confirm at the first prenatal ultrasound, which refines the date based on crown-rump length.
• If LMP is uncertain or cycles are irregular, let your care team know — an early ultrasound will provide the most reliable dating.
• View the due date as a reference window rather than a scheduled arrival. Most pregnancies deliver within ±2 weeks of the calculated date.