Pregnancy Week Calculator

How Gestational Age Is Counted

Gestational age is measured from the first day of your last menstrual period, not from the day you conceived. That convention surprises a lot of people: on the day of conception you are already counted as about 2 weeks pregnant. The reason is practical. Most people know when their last period started, while few can pinpoint the day an egg was fertilized, because ovulation happens roughly 2 weeks after the period begins and the fertile window itself spans several days.

Counting works the same way at every stage. Take the number of whole days that have passed since the first day of the last period, divide by 7, and the whole number is the completed weeks. The remainder is the extra days. A pregnancy that started 100 days ago sits at 14 weeks 2 days, because 100 / 7 = 14 with 2 left over. Clinics write this as 14w2d, and the day count never resets at the end of a week, so the day after 13 weeks 6 days is 14 weeks 0 days.

Fetal age, sometimes called embryonic age or conceptional age, is the time since fertilization. It runs about 2 weeks behind gestational age for a textbook 28 day cycle with ovulation on day 14. At 14 weeks 2 days gestational age, the fetus itself is about 12 weeks old. Medical records, ultrasound reports, and appointment scheduling all use gestational age, which is why the 40 week figure covers a period that includes roughly 2 weeks before conception was even possible.

The 280 Day Method and Naegele's Rule

The standard estimate adds 280 days to the first day of the last menstrual period. That number equals 40 weeks of 7 days, and it assumes ovulation happened on day 14 of a 28 day cycle. Franz Naegele popularized a shortcut for the same arithmetic in the early 1800s: add 1 year and 7 days to the LMP, then subtract 3 months. Both routes land on the same date, and the day-counting version is easier to check with a calendar or a calculator.

The American College of Obstetricians and Gynecologists, together with the Society for Maternal-Fetal Medicine and the American Institute of Ultrasound in Medicine, endorses this method as the starting point when the last period is certain and the cycle is regular. Their guidance, Committee Opinion No. 700 published in 2017 (see acog.org), treats the LMP estimate as the baseline and reserves changes for cases where an ultrasound disagrees enough to matter, which the next section covers.

Worked forward from January 1, 2026, the estimate lands on October 8, 2026: 280 days later. Run backward, a due date of October 8, 2026 implies an LMP of January 1, 2026. The symmetry is the whole point. One date determines the other, so a calculator that accepts either input can fill in the rest.

Working Backward From a Due Date

Most people learn their due date at a first appointment, often after an early ultrasound, before they ever think in terms of weeks. The reverse calculation takes whatever date remains between today and the due date and subtracts it from 280. With 180 days left until the due date, the current count is 280 - 180 = 100 days, which is 14 weeks 2 days.

The reverse direction is also how due-date math checks out in the other order. A due date of October 8, 2026 minus 280 days gives January 1, 2026 as the implied LMP. If that implied date cannot possibly be right, for example if it falls after a period you in fact had, then the due date itself came from an ultrasound adjustment, and the weeks count should follow the ultrasound rather than any remembered period.

One caution applies to the reverse direction. Because the arithmetic assumes a 280 day pregnancy, a due date that was already adjusted by a clinician carries that adjustment inside it. Recomputing the LMP from an adjusted due date will not reproduce your calendar period, and that is expected. The weeks count is still valid; only the implied LMP drifts.

When Cycle Length Shifts the Estimate

The 280 day method quietly assumes ovulation on day 14. Cycles that run longer usually ovulate later, which moves conception and the true due date later by about the same number of days. The common adjustment adds the difference between your cycle length and 28 days to the estimate. A 30 day cycle shifts the due date 2 days later, because 30 - 28 = 2, so the same January 1, 2026 LMP points to October 10, 2026 instead of October 8, 2026. A 26 day cycle pulls it 2 days earlier.

The adjustment is an approximation, not a law. Cycle length varies from month to month, and the day of ovulation within a cycle varies too. That is why clinical practice leans on first trimester ultrasound rather than cycle arithmetic when the two disagree. The NIH MedlinePlus review of calculating due dates describes the same 280 day convention. A calculator that accepts a cycle length is most useful before any ultrasound exists, or for people tracking ovulation with test strips or temperature, where the actual ovulation day is known and beats any formula.

If you know the ovulation day instead, a decent approximation adds 266 days to it, since fetal age at birth averages about 38 weeks. A known ovulation date of January 15, 2026 points to a due date around October 8, 2026, the same answer the LMP route produced for a January 1 period with ovulation 2 weeks later.

Term Definitions from ACOG

The 40 week mark is the anchor for the estimate, but deliveries cluster across a 5 week window around it, and outcomes differ across that window. In 2013, a work group convened with ACOG and the Society for Maternal-Fetal Medicine replaced the single word term with four designations, based on evidence that respiratory and other neonatal outcomes vary within the old 37 to 42 week band. The categories, from Committee Opinion No. 579 (summarized on PubMed and health.state.mn.us), are:

• Early term: 37 weeks 0 days through 38 weeks 6 days
• Full term: 39 weeks 0 days through 40 weeks 6 days
• Late term: 41 weeks 0 days through 41 weeks 6 days
• Postterm: 42 weeks 0 days and beyond

Deliveries before 37 weeks 0 days are preterm. The labels matter because the old habit of treating everything from 37 weeks onward as equally ready no longer holds. Elective deliveries scheduled without a medical reason are generally avoided before 39 weeks 0 days for that reason, and research published around the 2013 redefinition documented higher respiratory morbidity for newborns at 37 weeks than at 39 or 40.

So when a calculator says 40 weeks, that is the center of the estimate, not a deadline. The American College of Obstetricians and Gynecologists guidance from 2013 and reaffirmed in later documents treats the due date as the midpoint of a normal range, with the exact wording above used on birth certificates and in research.

Trimesters and Key Week Milestones

The 40 weeks divide into three trimesters. The first runs from week 1 through 13 weeks 6 days, the second from 14 weeks 0 days through 27 weeks 6 days, and the third from 28 weeks 0 days until delivery. The boundaries follow clinical convention rather than a biological switch, but they line up with appointment schedules, screening windows, and the point at which most early symptoms ease off.

The anatomy scan window deserves a note. The detailed mid-pregnancy ultrasound is usually scheduled between 18 and 20 weeks because that is when structures are big enough to measure and there is still room for follow-up if something needs a second look. The 24 week viability figure describes where neonatal survival becomes statistically likely with intensive care; it is a population statistic, not a promise, and outcomes at that age vary widely by hospital and by week.

Ultrasound Dating and When Dates Get Revised

An ultrasound in the first trimester, up to and including 13 weeks 6 days, is the most accurate tool for dating a pregnancy. Measurement of the crown rump length in that window predicts the due date within about 5 to 7 days. Second trimester measurements drift wider, roughly 7 to 10 days, and third trimester measurements drift wider still, which is why a late pregnancy scan that suggests a different date rarely triggers a change. ACOG's Committee Opinion No. 700 from 2017 (indexed at PubMed) sets out redating rules: the further along the pregnancy, the larger the disagreement has to be before the due date is revised.

The practical sequence looks like this. The LMP estimate comes first, the first ultrasound either confirms it or replaces it, and after that the date generally stays put. Subsequent changes are reserved for rare circumstances and need clear documentation. In vitro fertilization pregnancies skip the estimates entirely; the embryo transfer date gives an exact gestational age, and the due date is calculated from that.

For anyone using this calculator alongside clinical care, the advice is to treat the LMP number as a starting estimate. If your provider redated the pregnancy after an ultrasound, enter the ultrasound-based due date and let the reverse mode handle the weeks. The two inputs converge on the same arithmetic once the due date is fixed.

Common Mistakes When Counting Weeks

The count from the last period trips people up in predictable ways. The most frequent error is counting from conception, or from a positive test, which understates gestational age by about 2 weeks. A test turns positive roughly 10 to 14 days after conception, and by then the gestational count already sits at about 4 weeks. Anyone who says 2 weeks pregnant on the day of a positive test is using fetal age, not the gestational age their clinic will use.

Month-based reckoning causes its own confusion. Forty weeks is about 9.2 calendar months, and the lunar month shortcut of 10 months of 28 days describes the same span differently. None of these descriptions change the underlying day count, which is why every clinical tool works in weeks and days. Finally, a note on expectations: only a small share of deliveries happen on the estimated date itself. Most arrive within the 2 weeks on either side of it, which is exactly the spread the term definitions above describe.