Naegele's Rule from LMP
Estimated Due Date
Reference DateMay 28, 2027
Multiple valid formulas exist — results are estimates, not definitive answers
CalculatorDue Date Calculator — Pregnancy Wheel, Trimester & Milestones
Estimate your due date from last menstrual period, conception date, or IVF transfer. Includes trimester tracker, week-by-week progress bar, 40-week milestone timeline, and prenatal reference notes. Free, no signup. For general educational reference only.
Calculator
Your Pregnancy Details
First day of your last period. Naegele's Rule adds 280 days (40 weeks) from this date.
days
Due date estimate uses a fixed 280-day term and is not adjusted by cycle length. Only your prenatal care provider can confirm dating after an ultrasound.
Your Pregnancy Timeline
Trimester
Weeks 1–13
Current
of 40 weeks (280 days)
Days Until Due
266
Estimated ±2 weeks
Week 15% completeWeek 40
0
4
8
12
16
20
24
28
32
36
40
Pregnancy Milestones
✓
Conception
Week 0
2
Embryo Implants
Week 2
6
Heartbeat Detectable
Week 6
12
NT Scan
Week 12
20
Anomaly Scan
Week 20
24
Viability Threshold
Week 24
28
Third Trimester Begins
Week 28
37
Full Term
Week 37
40
Due Date
Week 40
The formula
How it’s calculated
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)
Due Date = LMP + 1 Year − 3 Months + 7 Days
Gestational Age (days) = Today − LMP
The classic 1812 Naegele formula assumes a 28-day cycle with ovulation on day 14, and counts a full 40 weeks (280 days) from LMP to due date. Modern textbooks still reference it because it is simple and accurate enough as a first-pass estimate for people with regular cycles.
Formula Source
This calculator uses the **Naegele's Rule for estimated due date (280 days from LMP)** from **Franz Naegele** published in **1812**.
Last Verified: 2026-07-30
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
Due Date = Transfer Date + 266 days − embryo_age_days
Example: a 5-day blastocyst transferred on March 1 → due date = March 1 + 261 days. A 3-day cleavage-stage embryo = March 1 + 263 days. The subtraction accounts for the days the embryo already developed outside the body before transfer, so gestational age matches standard LMP dating at the time of the first ultrasound.
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.
Health guide
Reading your results, kindly
Reading the Milestone Timeline
The horizontal milestone timeline marks commonly discussed reference points across 40 weeks of gestational age. Past milestones are shown with a solid completed check, the current or next upcoming milestone is highlighted in the accent color, and future milestones remain in an outlined state. These are reference markers, not deadlines or targets. Individual pregnancies progress along their own normal curve. If your care team uses a different dating after an ultrasound visit, that reference date supersedes calculator estimates — use the team dating for all subsequent scheduling and reference.
Progress Bar and Current Week
The 280-day progress bar divides the 40-week reference period into weekly ticks and fills based on your current gestational age. Think of it as a visual orientation, not a precise countdown. A reading of, for example, 22 weeks and 3 days means you are in your 23rd week of gestational age counting from LMP. Fetal developmental age measured from actual conception is roughly two weeks less than this. Birthing at 37–42 completed weeks is considered the term reference range. Deliveries prior to 37 weeks are commonly described as pre-term; those at 42 completed weeks and beyond are post-term.
What the Due Date Is Not
The due date is not a pick-up window for a package. It does not predict the exact day your baby will arrive. It is a scheduling anchor — the date your care team uses to time screening visits, laboratory panels, growth checks, and induction conversations. It is also the frame most apps and books use for developmental week-by-week descriptions. Fixating too closely on the exact date often adds unnecessary stress in the final weeks. A full two weeks on either side is entirely normal and expected.
When to Reach Your Care Team Promptly
A due-date calculator is an educational reference, not a substitute for prenatal care. Reach your prenatal care team promptly for any of the following at any point in your pregnancy: persistent or worsening abdominal or pelvic discomfort, vaginal bleeding or unusual fluid loss, severe or unrelenting headache, visual changes, significant swelling of the hands or face, fever, reduced fetal movement you notice after 28 weeks, painful or frequent contractions before 37 weeks, or any other change that genuinely worries you. Do not wait for a milestone date or a scheduled visit to bring up something that feels wrong.
Nutrition, Movement, and Rest
Prenatal nutrition, gentle movement, adequate hydration, and consistent sleep support your comfort and your baby development across all three trimesters. A generally balanced pattern that includes sufficient protein, iron, folate, DHA, calcium, and fiber — with continued prenatal vitamin or provider-recommended supplement use — is a reasonable baseline. Regular walking, swimming, prenatal yoga, and strength work guided by someone trained in pregnancy can help with back comfort, circulation, energy, and preparation for labor. That said, every pregnancy is different; some people need more rest, others more movement. Follow the specific guidance your prenatal care team gives you, especially if you have any restrictions or body patterns they are monitoring.
FAQ
Frequently Asked Questions
The most common method is the Naegele rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If you know your conception date, the calculator adds 266 days instead. For IVF transfers, the due date is the transfer date plus 266 days minus the embryo age in days. Each method gives a slightly different estimate because they count from different starting points. Due date estimates have variation of plus or minus 2 weeks; only your prenatal care provider can confirm your dating, often using a first-trimester ultrasound.
Due date estimates from LMP alone have typical variation of roughly plus or minus 1 to 2 weeks for people with regular 26 to 32-day cycles. The estimate gets much more precise if it is confirmed or adjusted by a first-trimester ultrasound (roughly 7 to 13 weeks) using crown-rump length, which narrows variation to about plus or minus 3 to 5 days in that window. For this reason, many prenatal care teams use the first-trimester ultrasound dating as the official reference date if it differs from LMP dating by more than a set threshold. Only about 5 percent of birthing people deliver spontaneously on their calculated due 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. For IVF, the calculator subtracts the embryo age (3 or 5 days) from the 266-day total to account for development outside the body.
A simple way to count at home is to add 7 days from your LMP date for each week. Pregnancy weeks are counted as completed weeks, so 8 weeks and 3 days means you are currently in your 9th gestational week. Note that gestational age (used universally by prenatal care teams and this calculator) counts from the first day of your last menstrual period, about two weeks before actual conception. Fetal age counts from the actual day of fertilization and is therefore about two weeks less than gestational age. So when the calculator says 20 weeks, the fetal developmental age from actual conception is approximately 18 weeks.
A full term pregnancy is generally considered to run from 37 completed weeks through 42 completed weeks of gestational age. Deliveries prior to 37 weeks are commonly described as pre-term, while those at 42 completed weeks and beyond are described as post-term. The due date itself is a single reference point within this wider term window, not a deadline. Going one or even two weeks past the calculated due date is within the normal range for first pregnancies and for many subsequent pregnancies. Care teams typically schedule increased monitoring starting around week 40 and induction conversations usually begin around 41 weeks.
If your cycles are irregular, longer than 35 days, or you do not remember your LMP clearly, the LMP-based Naegele estimate loses reliability. Prefer a conception-date estimate if you know ovulation or a specific high-probability intercourse window, or an IVF-transfer estimate if applicable. Otherwise, your first-trimester ultrasound dating will become the most accurate anchor you have. Your prenatal care team will use that ultrasound date for the rest of the pregnancy timeline. Do not rely on LMP math if you know your cycles vary significantly, and always confirm your dating with a qualified prenatal provider.
Care teams commonly recalibrate the official due date after the first-trimester ultrasound because crown-rump length in that window is the most accurate population-level predictor of gestational age we have. If the ultrasound date and your original LMP date differ by more than the local threshold (often about 5 to 7 days in the first trimester), the ultrasound date usually becomes the reference. This is normal and common — it does not mean anything is unusual about your pregnancy, and it does not mean the baby is ahead or behind. It simply gives you a more accurate anchor for the rest of your prenatal scheduling.
Going one or even two weeks past the calculated due date is within the normal range for first pregnancies and for many subsequent pregnancies. Care teams typically schedule increased monitoring (non-stress tests, fluid checks, occasional sonographic fetal-wellness surveys) starting around week 40. Induction conversations usually begin around 41 weeks, and induction is typically recommended by 42 weeks to reduce rare perinatal risk associated with advanced gestational age. If you are past your date, the most important next step is a direct conversation with your prenatal care team about their monitoring plan and your preferences for the path forward.
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