Pregnancy Due Date Calculator
Estimate your due date and current week, updates as you pick a date.
How This Is Calculated
This calculator uses Naegele's rule, the standard method used to estimate a due date: adding 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle with ovulation around day 14. If you enter a conception date instead, it adds 266 days (38 weeks), since conception typically occurs about two weeks after the start of the last period. Along with the due date, the tool also shows current gestational age in weeks and days, which trimester that falls in, and the calculated end dates of the first and second trimesters.
Why the Estimate Can Be Off
Naegele's rule assumes a standard cycle length, if your cycles are longer or shorter than 28 days, your actual ovulation and conception timing shifts, which is one reason ultrasound dating (especially in the first trimester) tends to be more precise than date-based calculation alone.
Worked Example
If the first day of the last menstrual period was July 5, adding the standard 280 days lands on a due date of April 11 the following year. Using today's date to check current progress, the gap between the last period and today comes out to 30 days, which converts to 4 weeks and 2 days of gestational age, placing that pregnancy in the first trimester (which runs through 13 weeks). The calculator also projects two milestone dates directly from the last period: the end of the first trimester falls 13 weeks (91 days) after the last period, and the end of the second trimester falls 27 weeks (189 days) after it. If a conception date of January 1 were entered instead of a last-period date, the calculator adds 266 days rather than 280, landing on a due date of September 24 that same year, 14 days earlier than the 280-day figure would suggest for an equivalent starting point, reflecting that conception itself already occurred roughly two weeks after what would have been the last period.
Choosing Between the Last Period and Conception Date Method
Most people use the "Last Period" tab, since the first day of the most recent menstrual period is generally easier to recall precisely than an exact conception date, particularly since conception can't usually be pinpointed to a single day without fertility tracking or a known ovulation date. The "Conception Date" tab is more useful for pregnancies conceived through IVF or other assisted reproduction, where the exact conception or embryo transfer date is medically documented, or for people who have been closely tracking ovulation and are confident in the exact date. Both tabs ultimately estimate the same underlying due date, they just start from a different, equally valid reference point in the standard 280-day gestational timeline.
The History and Logic of Naegele's Rule
Naegele's rule is named after Franz Naegele, a 19th-century German obstetrician who popularized the standard method for estimating due dates, based on the observation that human pregnancy averages close to 280 days from the last menstrual period in women with typical 28-day cycles. The calculation is often described as a simple formula: take the first day of the last period, subtract three months, and add seven days (and one year, if needed), which produces the same result as adding 280 days directly. Despite being nearly two centuries old, the rule remains the standard starting point for due date estimation in modern obstetric care, though it's now typically combined with first-trimester ultrasound measurements for greater accuracy, particularly for people with irregular cycles where the last-period date alone is a less reliable anchor.
Why Only 5% of Babies Arrive on Their Due Date
Despite how precisely a due date can be calculated to a specific day, actual birth timing naturally varies across a range of roughly two weeks before to two weeks after that calculated date for the large majority of pregnancies. This variation comes from several sources: individual differences in typical pregnancy length, uncertainty in exactly when ovulation and conception occurred even with a known last-period date, and the many biological factors that influence when labor actually begins. This is why medical professionals treat the due date as the statistical center of a range rather than a firm deadline, and why full-term birth is defined as any birth between 37 and 42 weeks of gestational age rather than a single target week.
How Ultrasound Dating Improves on Naegele's Rule
Naegele's rule works entirely from a remembered date and an assumed 28-day cycle, both of which introduce potential error, not everyone tracks their last period precisely, and cycle length varies naturally between individuals and even between cycles for the same person. First-trimester ultrasound dating instead directly measures the size of the developing embryo or fetus, most commonly the crown-rump length in early pregnancy, and compares it against standardized growth charts to estimate gestational age. Because fetal growth in the first trimester is remarkably consistent across pregnancies, this method is generally considered more accurate than date-based calculation alone, particularly for people with irregular cycles, uncertain last-period dates, or pregnancies conceived through fertility treatment where the conception date may already be precisely known. This is why a due date estimated by this calculator is often adjusted after a first ultrasound, sometimes by several days in either direction.
Recalculating as a Pregnancy Progresses
Because the due date is fixed once calculated from a given last-period or conception date, there's no need to recalculate it repeatedly, but the current gestational age figure naturally changes daily since it's measured against today's date. Revisiting this calculator periodically through a pregnancy gives an updated week-and-day count without needing to manually track the math, useful for comparing against prenatal appointment schedules, screening windows, or simply keeping track of overall progress toward the due date. If a healthcare provider updates the due date based on ultrasound measurements, using that new provider-confirmed date going forward rather than the original last-period-based estimate keeps subsequent gestational age figures aligned with your actual medical record.
Milestones Beyond the Trimester Markers
Beyond the trimester boundaries this calculator highlights, pregnancy is typically discussed in terms of specific gestational weeks tied to common prenatal care milestones, the first trimester screening window (roughly weeks 11 to 13), the anatomy ultrasound (typically around weeks 18 to 22), the point of viability outside the womb (around week 24, though outcomes improve significantly with each additional week after that), and the full-term range beginning at week 37. Knowing current gestational age precisely, which is what this calculator's "Current Gestational Age" figure provides, makes it easier to understand which stage of prenatal care and fetal development corresponds to where a pregnancy currently stands.
Frequently Asked Questions
How many weeks is a full-term pregnancy?
Full term is typically considered 37 to 42 weeks, with 40 weeks being the average reference point used for due date calculations. Babies born anywhere in that range are generally considered full-term.
Should I trust this over my doctor's estimate?
No, always prioritize your healthcare provider's dating, which typically incorporates ultrasound measurements and is more accurate than date-based calculation, especially for irregular cycles.
Why does the calculator add 280 days for last period but 266 for conception date?
Pregnancy is conventionally measured in gestational weeks starting from the first day of the last menstrual period, even though conception typically happens about two weeks later, around ovulation. A 280-day (40-week) pregnancy measured from the last period therefore corresponds to roughly 266 days (38 weeks) from the actual conception date, which is why the calculator uses a shorter offset when you enter a conception date directly.
What is gestational age and how is it different from fetal age?
Gestational age is the standard measurement used throughout pregnancy and counts from the first day of the last menstrual period, which is roughly two weeks before conception actually occurs. Fetal age (or conceptional age) counts from the actual date of conception instead, and is therefore about two weeks less than gestational age at any given point. Medical dating, due dates, and trimester boundaries are virtually always expressed in gestational age, which is what this calculator reports.
Why do the trimester boundaries fall at 13 and 27 weeks?
A full 40-week pregnancy is commonly divided into three roughly equal trimesters: the first trimester runs through the end of week 13, the second trimester runs from week 14 through the end of week 27, and the third trimester covers week 28 onward until birth. These boundaries are a widely used convention rather than a strict biological cutoff, and some sources define them with slightly different week ranges.
What percentage of babies are actually born on their due date?
Only around 5% of babies are born on their exact calculated due date. Most births occur within a window of about two weeks before to two weeks after the estimated date, which is why medical guidelines generally define full term as a range, 37 to 42 weeks, rather than treating the calculated due date as an exact deadline.