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Find out when your baby is due

Enter the first day of your last menstrual period and this free pregnancy due date calculator instantly shows your estimated due date, how many weeks pregnant you are today, the exact dates of each trimester, and a week-by-week milestone timeline — from the first detectable heartbeat to full term. A progress bar shows how far through the 40 weeks you've come.

Five calculation methods cover every situation: last period (with cycle-length adjustment), conception date, IVF embryo transfer (day-3, day-5 or day-6, fresh or frozen), ultrasound dating, and a reverse due date calculator that works backwards from a known due date to the likely conception window.

How due dates are calculated

The medical standard is Naegele's rule: your due date is 280 days — exactly 40 weeks — after the first day of your last period. If you know the conception date, it's conception + 266 days instead, because gestational counting includes the ~2 weeks before ovulation. IVF pregnancies are dated most precisely of all: a day-5 blastocyst transfer is due 261 days after transfer, a day-3 embryo 263 days after.

One assumption matters: Naegele's rule presumes a 28-day cycle with ovulation on day 14. If your cycles run longer or shorter, pick your average cycle length and the calculator shifts the date accordingly — a 35-day cycle moves the due date about a week later than the naive formula. For irregular cycles, the ultrasound tab (enter the scan date and the gestational age it reported) gives the most reliable answer.

How far along am I?

Alongside the due date, the calculator answers the question everyone actually asks: how many weeks pregnant am I right now? You'll see your gestational age in weeks and days, the week of pregnancy you're in, your current trimester, days remaining, and the estimated conception date. Trimester boundaries follow the standard convention — the second trimester begins at 14w0d and the third at 28w0d — each converted to real calendar dates.

The milestone list maps the journey ahead: heartbeat detectable around 6 weeks, the anatomy scan window near 20 weeks, viability at 24 weeks, early term at 37, full term at 39, and the 42-week post-term limit. Past milestones are checked off automatically. Apps like Flo and Ovia show similar timelines but lock the detail behind an account registration.

How this compares to BabyCenter, The Bump, Flo and Ovia

The major pregnancy platforms all use the same Naegele's rule math — so the due date itself is identical. The differences are in what surrounds it. BabyCenter and WhatToExpect.com require an account to save results and immediately begin email campaigns. Flo and Ovia are mobile apps that collect your health data and share or monetize it with partners. The Bump and Pampers prompt for an email to "send your results" — results you've already seen on screen.

This calculator skips all of that. Your dates are entered, calculated, and displayed entirely in your browser. Nothing is uploaded. There's no account, no email prompt, no push to download an app. Results are yours the moment you enter a date, and a shareable link lets you send them to a partner or provider without handing any data to a server. For users who want the same information without the tracking, this is the fastest and most private option available.

A due window, not just a date

Only about 4–5% of babies are born on their due date. Roughly 80% arrive within two weeks either side. That's why this tool always shows the full 37–42 week window around your date — a more honest picture of when to actually expect your baby. Expecting twins? Tick the twins option and you'll see a ~37-week estimate, since twin pregnancies usually deliver earlier. Most other online calculators, including the ones on The Bump and Pampers, don't offer a twins mode at all.

For fun, the result includes your due date's day of the week, zodiac sign, birthstone and birth flower — without the email wall the big pregnancy sites put in front of them.

Private by design — your dates never leave your device

Period and pregnancy dates are sensitive health data. Unlike the major pregnancy portals — which funnel your dates through their servers and into email-capture funnels and app downloads — this calculator runs 100% in your browser. Nothing is uploaded, nothing is stored, and there is no account, ever. Your result auto-saves locally on your own device so it's still there when you return, and the optional share link encodes the dates only in the URL itself.

Works on iPhone, Android, tablet or desktop without installing anything. When you're ready for the next step, UtiloKit's Age Calculator, Date Difference Calculator and Countdown to Date tools are handy companions — count down the days to the big arrival.

Naegele's Rule: the 200-year-old formula behind every due date

Franz Karl Naegele (1778–1851), a German obstetrician practicing in Heidelberg, formalized the rule that became the global obstetric standard. His formula: take the first day of the Last Menstrual Period (LMP), add one year, subtract three months, and add seven days — which works out to LMP + 280 days (40 weeks). For example, an LMP of August 1 becomes August 8, then back to May 8, then forward one year to May 8 of the following year. The rule is not counted from conception because conception was rarely known in the 19th century and, crucially, because the LMP is almost always remembered while the ovulation date is not.

The 280-day figure is not arbitrary. The average gestation from conception to birth is approximately 266 days (38 weeks). Adding 14 days to account for the assumption that ovulation — and therefore conception — occurs on day 14 of a standard 28-day cycle brings the total to 280. This is where Naegele's rule begins to break down for many women: the formula presumes a textbook 28-day cycle and mid-cycle ovulation. A woman with a 35-day cycle typically ovulates around day 21, not day 14, meaning her true due date is roughly seven days later than the unadjusted formula suggests. Conversely, a 21-day cycle moves the date seven days earlier. Cycle-length adjustment — shifting the due date by (cycle length − 28) days — corrects for this, which is exactly what this calculator does when you specify your average cycle length.

Despite its age and assumptions, Naegele's rule remains the foundation of every due date calculator in existence, from hospital EHR systems to BabyCenter to this tool. Its endurance reflects a practical reality: the LMP is the one piece of timing data almost every pregnant person knows, it requires no medical equipment, and population-level studies have validated 280 days as a reasonable average. The rule was never meant to predict the day of birth — only to anchor a clinical timeline for monitoring fetal development, scheduling screening tests, and identifying pregnancies that may be progressing differently from average.

Ultrasound dating: why a scan often overrides the calendar

The most accurate dating method available in modern obstetrics is a first-trimester ultrasound measuring crown-rump length (CRL), performed between 8 and 13 weeks of gestation. During this narrow window, embryo size follows exceptionally predictable mathematical growth curves, allowing gestational age to be estimated to within ±5–7 days — meaningfully better than LMP-based dating, which can be off by two weeks or more in women with irregular cycles or an uncertain LMP. The CRL measurement is straightforward: the sonographer captures the longest straight-line dimension of the embryo from the top of the head to the base of the spine (excluding the legs), and a reference table or formula converts that millimetre measurement into a gestational age.

The American College of Obstetricians and Gynecologists (ACOG) has clear guidance on when ultrasound takes precedence over LMP. If the LMP-based due date and the ultrasound-based due date disagree by more than 7 days in the first trimester, or more than 10 days in the second trimester, the ultrasound date should be used — and once established, that due date is not typically changed again even if later scans suggest a different age. This policy reflects the fact that early fetal growth is tightly regulated by genetics and biology, whereas LMP recall and cycle-length assumptions introduce human error. An increasing number of hospitals and midwifery practices now routinely offer a dating scan at 8–10 weeks precisely to eliminate LMP uncertainty before the first-trimester combined screening at 11–13 weeks.

For fetal growth assessment later in pregnancy, many centres are moving from older Hadlock reference charts (derived from US populations in the 1980s) to the INTERGROWTH-21st standard — an international fetal growth reference developed from eight countries across four continents, designed to represent optimal growth rather than population averages. INTERGROWTH-21st is increasingly used for both dating and growth surveillance, particularly in settings where the local population differs significantly from the demographics of the original Hadlock study. By the third trimester, however, ultrasound dating becomes far less precise — third-trimester biometric measurements can diverge by three to four weeks in either direction, which is why providers rely on the first-trimester date throughout the pregnancy rather than revising it based on later scans.

Gestational age vs embryonic age: why '6 weeks pregnant' means a 4-week embryo

Obstetric care worldwide uses gestational age — weeks counted from the first day of the Last Menstrual Period — as the standard measure of pregnancy. Embryologists and fertility specialists, by contrast, use embryonic age (also called fetal age or post-conceptional age), which is counted from the moment of fertilization. Because ovulation and fertilization typically occur roughly 14 days after the LMP, embryonic age runs approximately two weeks behind gestational age at all times. A woman told she is "6 weeks pregnant" by her obstetrician has an embryo that is only approximately 4 weeks old from the moment of conception.

This discrepancy causes genuine confusion, particularly for people who know exactly when they conceived — such as IVF patients or those who tracked ovulation precisely. The terminology is not an error or an approximation; it is a deliberate convention adopted because the LMP is reliably recalled while fertilization is almost never directly observed in natural conceptions. The entire clinical infrastructure of prenatal care — screening timelines, anatomy scans, growth benchmarks, and delivery planning — is built on gestational weeks, making it impractical to switch conventions. Understanding the offset matters practically: a patient who conceived through IVF on a known date and uses that date in an LMP-based calculator without adjustment will get a due date two weeks too late.

The trimester boundaries follow gestational age by convention: the first trimester spans gestational weeks 1 through 13 (the period of organogenesis and the highest miscarriage risk); the second trimester covers weeks 14 through 27 (rapid fetal growth, the anatomy scan, and viability threshold at 24 weeks); the third trimester runs from week 28 to birth. At 40 weeks gestational age — the nominal due date — the baby has been developing for approximately 38 weeks from conception. This means that at birth, even a "full-term" newborn is developmentally equivalent to a 38-week-old embryo, not a 40-week-old one. The distinction has no practical impact on postnatal care, but it is worth understanding when interpreting pregnancy literature that uses the two systems inconsistently.

Preterm, full term, and post-term: why the definitions were redrawn

For most of the 20th century, any birth at or after 37 completed weeks was considered "full term" and assumed to carry similar outcomes regardless of exactly when in that range it occurred. In 2013, the American College of Obstetricians and Gynecologists (ACOG) formally redrew the terminology after accumulating evidence showed that neonatal outcomes differ meaningfully across the weeks once labelled simply "term." The revised definitions divide the near-term period into four categories: early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks), and post-term (42 weeks and beyond). Births before 37 weeks remain classified as preterm, with sub-categories for late preterm (34–36 weeks), very preterm (28–33 weeks), and extremely preterm (under 28 weeks).

The motivation for this change was data demonstrating that babies born at 37–38 weeks have measurably higher rates of respiratory distress, NICU admission, feeding difficulties, and temperature instability compared to those born at 39–40 weeks. The previous "37 weeks is fine" guideline had contributed to a pattern of elective inductions and scheduled caesarean sections at 37–38 weeks for non-medical reasons — reducing risk for providers managing scheduling, but raising risk for neonates who could have benefited from two more weeks of lung maturation and brain development. The 2013 redefinition, and ACOG's subsequent guidance discouraging non-medically indicated deliveries before 39 weeks, measurably reduced early-term elective deliveries in the United States over the following years.

Preterm birth — delivery before 37 completed weeks — occurs in approximately 10–10.5% of US births, one of the highest rates among high-income nations. The major contributors include spontaneous preterm labour (often associated with intrauterine infection or inflammation), multiple gestations (twins and higher-order multiples have dramatically shorter average gestational lengths — twins average 36–37 weeks, triplets 33–34 weeks), placental abnormalities such as placenta previa or abruption, cervical incompetence, and a history of prior preterm birth, which is one of the strongest individual predictors of recurrence. Post-term pregnancy (beyond 42 weeks) carries its own risks — placental insufficiency and meconium aspiration among them — which is why most providers offer induction by 41–42 weeks rather than allowing pregnancy to continue indefinitely past the due date.