Due Date Calculator
The most common question after a positive pregnancy test is simple: when is the baby actually due? This calculator answers it directly from the first day of your last menstrual period, using the standard method doctors and midwives use, along with the key milestone dates that mark each stage of pregnancy along the way.
How the Due Date Calculator Works
Enter the first day of your last menstrual period and your average cycle length, and the calculator adds 280 days (40 weeks) to that date, adjusting slightly for cycle lengths that differ from the standard 28-day assumption. Alongside the due date itself, it returns your estimated conception date, current gestational age, and the boundaries of each trimester and full-term milestone.
Naegele’s Rule: The Standard Behind This Calculation
This calculator applies Naegele’s rule, the standard method for estimating a due date from the last menstrual period, first described in the early 1800s and still the basis for pregnancy dating today: add one year, subtract three months, and add seven days to the first day of the last period — mathematically equivalent to simply adding 280 days. This method assumes a 28-day cycle with ovulation occurring on day 14; this calculator adjusts the calculation slightly for cycle lengths that differ from that 28-day standard, since a longer or shorter cycle shifts the likely ovulation and conception date correspondingly.
Why the Due Date Is an Estimate, Not a Fixed Appointment
It’s worth setting realistic expectations from the start: only about 4% of babies are actually born on their calculated due date. A due date represents the statistical center of a normal delivery range, not a precise prediction — full-term delivery spans roughly 37 to 42 weeks, meaning a healthy delivery could reasonably happen anywhere across several weeks around the calculated date. Treating the due date as the middle of a probable window, rather than a specific day to circle on the calendar, matches how healthcare providers actually think about it.
Why Cycle Length and Regularity Affect Accuracy
This calculation is most accurate for someone with a fairly regular cycle close to the standard 28-day length, since the entire method rests on the assumption that ovulation happens around day 14 of a predictable cycle. For those with irregular cycles — common with certain conditions — LMP-based dating becomes less reliable, and an early ultrasound measuring the fetus directly is generally considered a more accurate dating method in that situation, since it measures actual physical development rather than relying on cycle-based assumptions. If you have an ultrasound-based estimate that differs somewhat from this calculator’s LMP-based result, the ultrasound dating is often given more weight, particularly for those with known irregular cycles.
Current Gestational Age: Where You Are Right Now
Beyond the due date itself, this calculator’s gestational age figure tells you how far along the pregnancy currently is, expressed in weeks and days — the standard way pregnancy progress is measured and communicated in prenatal care. This figure counts from the last menstrual period, not from the estimated conception date, which is exactly why gestational age at the moment of conception is already counted as roughly 2 weeks, even though conception itself happens after LMP.
Trimester Boundaries and What Changes at Each Stage
This calculator marks the standard trimester boundaries — first trimester through week 13, second trimester through week 27, and third trimester onward to full term at 37 weeks. Each trimester is associated with different typical developmental milestones and different standard prenatal monitoring schedules, though your specific care plan should always come from your actual healthcare provider rather than a general calculator, since individual circumstances can shift what monitoring or testing is recommended at any given stage.
Tracking Recommended Weight Gain Through Pregnancy
Once you know your current gestational age from this calculator, recommended weight gain guidance becomes directly relevant, since healthy gain targets are based specifically on how far along a pregnancy is combined with pre-pregnancy BMI. The Pregnancy Weight Gain Calculator uses your gestational week alongside pre-pregnancy BMI to estimate an appropriate weight gain range at your current stage.
If Your Dating Information Comes From a Different Source
This calculator uses last menstrual period as its starting point specifically, which is the most common and immediately available piece of information for someone who has just confirmed a pregnancy. If you instead know your specific conception date, or have an ultrasound-based gestational age estimate you’d rather calculate from, the Conception Calculator accepts all three of these different starting points and converts any of them into the same standardized due date and milestone information this calculator provides.
Before Pregnancy: Tracking Ovulation for Conception
If you’re not yet pregnant and are instead trying to identify the best timing for conception, this calculator’s forward-dating logic isn’t the relevant tool. The Ovulation Calculator is built specifically for that earlier stage, estimating an upcoming fertile window based on your cycle rather than calculating a due date from an already-established last period.
Age Calculations for Later Milestones
Once your due date arrives and the baby is born, tracking exact age going forward uses a related but distinct calculation from pregnancy dating. The Age Calculator picks up at that point, calculating precise years, months, and days from an actual birth date rather than an estimated due date.
Frequently Asked Questions
How is a due date calculated?
Using Naegele’s rule: adding 280 days (40 weeks) to the first day of your last menstrual period, assuming a standard 28-day cycle. This calculator adjusts slightly for cycle lengths that differ from that assumption.
How accurate is a due date calculated from the last period?
It’s a reasonable estimate for someone with a regular cycle close to 28 days, but only about 4% of babies are actually born on their exact calculated due date. Full-term delivery spans a range of roughly 37 to 42 weeks, so the due date represents the center of that probable window, not a precise prediction.
Why might an ultrasound-based due date differ from this calculator’s result?
Ultrasound dating measures the fetus’s actual physical development directly, which can be more accurate than LMP-based calculation, particularly for those with irregular menstrual cycles where the LMP method’s underlying assumptions are less reliable.
What is gestational age, and how is it different from the age of the actual embryo or fetus?
Gestational age is counted from the first day of the last menstrual period, not from the actual conception date, which happens roughly two weeks later. This means gestational age is always about two weeks ahead of the fetus’s actual developmental age from conception.
What should I do if I don’t know the date of my last period?
If you know your approximate conception date instead, or have an ultrasound-based gestational age estimate, the Conception Calculator accepts those alternative starting points and calculates the same due date and milestone information from whichever piece of information you actually have available.