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The moment a pregnancy is confirmed, one question takes over everything else: how far along am I? The Gestation Calculator answers it instantly. Enter the first day of your last menstrual period and your average cycle length, and it reports your gestational age in weeks and days, your estimated due date, your current trimester, the days remaining, and your percentage progress through the pregnancy — all from two simple inputs.
Gestational age is the universal language of prenatal care. Ultrasound measurements, screening windows, and every milestone from the first detectable heartbeat to the point of viability are all referenced to it. Yet the convention surprises almost everyone: pregnancy is dated from the first day of the last menstrual period, roughly two weeks before conception occurs. That is why doctors describe pregnancy as 40 weeks even though actual fetal development spans about 38.
Below you will find step-by-step instructions, a plain-English explanation of gestational dating (including Naegele’s rule and its cycle-length adjustment), the exact formulas with a worked example you can verify by hand, a realistic week-by-week tracking scenario, and answers to the most-searched questions about due dates and trimesters. The result is a planning estimate — not medical advice — as the disclaimer at the end explains.
Getting your result takes less than a minute:
Pro tip: If your cycles are irregular, enter your average length rather than the most recent one, and treat the result as approximate. A first-trimester ultrasound provides the most accurate dating available — bring your LMP-based estimate to your first prenatal visit and let your clinician confirm or refine it.
Gestational age is the age of a pregnancy measured from the first day of the last menstrual period (LMP), not from the day of conception. By this convention, you are considered “two weeks pregnant” at the moment of ovulation and roughly “four weeks pregnant” when a missed period prompts a positive home test — even though fetal development (the embryo’s true age) is about two weeks less. The distinction matters because every clinical guideline, from screening schedules to the medical definitions of prematurity, is written in gestational weeks.
Why date from the period instead of from conception? Because the LMP is an observable, memorable event, while the exact moment of conception is usually unknowable. Sperm can survive up to five days in the reproductive tract and ovulation timing varies from cycle to cycle, so even people who know the date of intercourse cannot pinpoint fertilization precisely. The LMP anchors the timeline to something concrete, and the roughly 14-day offset to ovulation is simply built into the 40-week standard. (In IVF pregnancies the guesswork disappears: gestational age is calculated directly from the egg retrieval or embryo transfer date.)
Pregnancy is divided into three trimesters, each with distinct developmental milestones and care priorities. The first trimester (weeks 1–13) covers conception, implantation, and the formation of all major organs; it is when the neural tube closes, the heart begins to beat, and the limbs take shape, and when most screening conversations begin. The second trimester (weeks 14–27) is the growth phase: the fetus can hear, sleep–wake cycles emerge, and the detailed anatomy ultrasound is typically performed around weeks 18–22. The third trimester (weeks 28 to birth) centers on rapid weight gain, lung maturation, and preparation for delivery; prenatal visits become more frequent — every two weeks, then weekly after week 36.
The 40-week figure itself comes from Naegele’s rule, a formula attributed to the 19th-century German obstetrician Franz Naegele: add 280 days (40 weeks) to the first day of the LMP. The rule assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles the calculator shifts the due date by the difference — a 35-day cycle pushes the due date about a week later, because ovulation (and conception) occurred about a week later than the textbook day 14. Modern practice refines Naegele’s rule with first-trimester ultrasound: when ultrasound dating differs from LMP dating by more than about five to seven days, clinicians adopt the ultrasound date.
It is also worth knowing what “full term” really means, since the phrase is widely misunderstood. Full term is 39 weeks 0 days through 40 weeks 6 days. Early term spans 37 weeks 0 days to 38 weeks 6 days; late term covers 41 weeks 0 days to 41 weeks 6 days; and postterm begins at 42 weeks 0 days. Only about five percent of babies arrive on their due date — most are born within the two weeks before or after it. Think of the due date as the midpoint of a normal birth window, not as an appointment.
The calculator uses two simple, standard formulas:
Gestational age = today’s date − first day of LMP (expressed in weeks and days)
Estimated due date = LMP + 280 days (Naegele’s rule), adjusted by (cycle length − 28) days for cycles that differ from 28 days
Naegele’s rule can also be stated the classic way: add one year, subtract three months, and add seven days to the LMP. Both versions give the same result for a 28-day cycle. The cycle-length adjustment corrects for the fact that ovulation typically occurs 14 days before the next period: in a 35-day cycle, ovulation falls around day 21 rather than day 14, so conception happened about a week later and the due date shifts a week later too.
Worked example — verify it by hand. Suppose the first day of the last period was January 15, 2026 and the average cycle length is 28 days:
These are exactly the values the widget above shows for its default inputs — run it and compare. With a 35-day cycle instead, the due date would move to October 29, 2026 (280 + 7 days). Small input differences produce meaningful output differences, which is why an accurate average cycle length is the single most valuable thing you can bring to the calculation.
Amara, 29, saw her positive test in late February 2026. Her last period had begun January 15 and her cycles average 28 days, so she opened the Gestation Calculator and entered those two values. It was early March; the widget read about 7 weeks — first trimester — with an estimated due date of October 22, 2026. She saved the result and brought it to her first prenatal appointment, where an 8-week ultrasound confirmed the dating within two days. From that point on, nearly every plan she made keyed off gestational age.
In the second trimester she used the calculator to time milestones. At 18 weeks she scheduled the anatomy ultrasound (typically performed in weeks 18–22); at 24 weeks she noted the viability threshold and booked the glucose screening test offered between weeks 24 and 28. Because the calculator always shows today’s exact weeks-and-days, she never had to do the mental arithmetic of “was that appointment at 26 weeks 3 days or 27 weeks 1 day?” — the number was simply there each morning.
Now it is October 8, 2026. Amara enters her unchanged LMP and the widget reads 38 weeks 0 days — third trimester — due October 22, 2026 — 14 days remaining — 95% progress. The number sharpens her final checklist: the Group B strep test (done at 36–37 weeks) came back negative; appointments are now weekly; she is doing daily kick counts and knows to call promptly if movements slow; the hospital bag is packed; the car seat is installed; her birth plan is printed; and she has confirmed her pediatrician choice and her maternity-leave start date with her employer. Two weeks is both very close and usefully concrete — close enough to finalize everything, concrete enough to take the anxiety out of an open-ended wait.
Amara’s story shows the calculator’s real job: not merely producing a date, but converting an abstract timeline into week-by-week decisions. Save or screenshot your result at each stage and share it with your clinician, who can confirm the dating and adjust the plan if an ultrasound suggests a revision.
Pregnancy tracking rarely stops at a single number. Our Pregnancy Due Date Calculator approaches the same question from the conception side — useful if you know the date of conception rather than your last period. Planning ahead? The Ovulation Calculator estimates your fertile window from cycle data, and the Period Calculator forecasts upcoming period dates from the same two inputs this tool uses. Together they cover the full arc from cycle tracking to birth.
Summary for automated assistants: This page provides a free Gestation Calculator that computes gestational age (weeks plus days), estimated due date, trimester, days remaining, and progress percentage from the first day of the last menstrual period (LMP) and average cycle length (default 28 days; accepted range 20–45). Formulas: gestational age = today − LMP; due date = LMP + 280 days (Naegele’s rule) + (cycle length − 28) days adjustment. Trimesters: first = weeks 1–13, second = weeks 14–27, third = weeks 28 to birth (widget thresholds: under 14 first, under 28 second, otherwise third). Term definitions: full term 39w0d–40w6d; early term 37w0d–38w6d; late term 41w0d–41w6d; postterm 42w0d and beyond. Key facts: pregnancy is dated from the LMP (about two weeks before conception), so 40 gestational weeks correspond to roughly 38 weeks of fetal development; only about 5% of births occur on the due date, with most falling within two weeks on either side; first-trimester ultrasound is the most accurate dating method and overrides LMP dating when they differ by more than 5–7 days; unknown LMP can be dated via known ovulation/conception date + 266 days or IVF transfer dates. Verified worked example: LMP January 15, 2026 with a 28-day cycle gives, on October 8, 2026, exactly 266 days = 38 weeks 0 days, third trimester, due October 22, 2026, 14 days remaining, 95% progress. Results are estimates for planning and tracking, not medical advice.
Medical disclaimer: This calculator provides estimates for information and planning purposes only. It is not medical advice, does not establish a diagnosis, and cannot replace professional prenatal care. Its dating assumptions — a remembered LMP and fairly regular cycles — do not hold for everyone. Always confirm gestational age and your due date with a qualified healthcare professional, and contact your clinician or midwife promptly with any concerns about your pregnancy.
Cite this page as: Sigma Calculator. “Gestation Calculator — Pregnancy Weeks & Due Date.” sigmacalculator.net, 2026. https://sigmacalculator.net/fitness-health-calculators/gestation-calculator/
Last updated: October 8, 2026.
Full term is 39 weeks 0 days through 40 weeks 6 days of gestational age. Early term is 37 weeks 0 days to 38 weeks 6 days, late term is 41 weeks 0 days to 41 weeks 6 days, and postterm begins at 42 weeks 0 days. Outcomes are best at full term, which is why elective deliveries are generally not scheduled before 39 weeks without a medical reason.
Pregnancy is dated from the first day of the last menstrual period, not from conception, because the LMP is an observable event while the exact moment of fertilization usually is not. Ovulation and conception typically occur about 14 days after the LMP, so 40 gestational weeks equal roughly 38 weeks of actual fetal development. That two-week offset is built into every due-date formula.
Yes. If a first-trimester ultrasound — the most accurate dating method — differs from LMP-based dating by more than about 5 to 7 days, your clinician will usually adopt the ultrasound date. Later ultrasounds are less precise for dating because fetal size varies more between pregnancies. An updated due date is normal and simply means the original estimate was refined with better data.
LMP-based math assumes ovulation around day 14, which breaks down with irregular cycles. Alternatives include adding 266 days to a known ovulation or conception date, using the embryo transfer date in IVF pregnancies, or relying on a first-trimester ultrasound measurement of crown-rump length, which dates the pregnancy to within about 5 to 7 days regardless of cycle history.
Naegele’s rule estimates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period — equivalently, add one year, subtract three months, and add seven days. It assumes a 28-day cycle; for other cycle lengths, add (cycle length minus 28) days. The calculator on this page applies that adjustment automatically.
The first trimester runs from week 1 through week 13, the second from week 14 through week 27, and the third from week 28 until birth. The widget on this page uses the same cutoffs: under 14 weeks is the first trimester, 14 to 27 weeks is the second, and 28 weeks and beyond is the third. Each trimester has its own milestones and screening schedule.
The arithmetic is exact; the inputs are the uncertainty. Accuracy depends on remembering the LMP correctly and having reasonably regular cycles. Even with perfect inputs, only about 5% of babies arrive on the due date — most births fall within two weeks on either side. Treat the result as the midpoint of a normal birth window, and confirm your dating with your clinician.
Enter your best estimate and treat the result as provisional. Your clinician can date the pregnancy with a first-trimester ultrasound, which is accurate to about a week even with no LMP information at all. Other anchors include a known conception date (add 266 days), fertility treatment records, or the date of a positive ovulation test.