Estimate your due date and current week of pregnancy from the first day of your last menstrual period.
This calculator uses Naegele's rule, the standard method most care providers use for an initial estimate: it adds 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is longer or shorter than the typical 28 days, the calculator adjusts slightly for that difference, since ovulation timing shifts along with cycle length.
Pregnancy is conventionally dated from the first day of your last period, not from the day of conception — which usually happens about two weeks later. That's why in week 4, for example, you're often only about two weeks past actual fertilization. This can feel confusing at first, but it's the standard convention doctors and ultrasounds use.
| Trimester | Weeks |
|---|---|
| First | 1–13 |
| Second | 14–27 |
| Third | 28–40+ |
Only about 5% of babies are born on their exact estimated due date — it's a helpful planning marker, not a precise prediction. Always confirm dates and milestones with your doctor or midwife, especially if your cycle is irregular.
Enter the first day of your last menstrual period, and optionally your average cycle length if it's different from the standard 28 days. Click Calculate, and the tool returns your estimated due date along with your current week and day of pregnancy. This gives you an immediate reference point for tracking development milestones and planning prenatal appointments around key stages of the pregnancy.
Naegele's rule, named after 19th-century German obstetrician Franz Naegele, is the standard formula still used today by most healthcare providers for an initial due date estimate: add 7 days to the first day of the last menstrual period, then subtract 3 months (or equivalently, add 280 days). It assumes a 28-day cycle with ovulation occurring on day 14. While simple, this method has remained in clinical use for roughly two centuries because it provides a reasonably reliable starting estimate for a population, even though any individual's actual delivery date can vary considerably from this calculated point.
Naegele's rule assumes a standard 28-day cycle, but many people have cycles that run somewhat longer or shorter. Since ovulation timing shifts along with overall cycle length — occurring later in longer cycles and earlier in shorter ones — a calculator that accounts for your actual average cycle length produces a more personalized estimate than the standard formula alone. If your cycles are notably irregular rather than just consistently longer or shorter than 28 days, this calculation becomes less reliable, and an early ultrasound is generally considered a more accurate method for establishing a due date in that situation.
The first trimester (weeks 1-13) covers the earliest and most rapid developmental stage, including the formation of major organs, though many early physical changes in the pregnant person aren't yet visibly obvious. The second trimester (weeks 14-27) is often described as the most comfortable stretch for many people, as early symptoms like nausea often ease and noticeable fetal movement typically begins. The third trimester (weeks 28-40+) involves the most significant growth in fetal size and weight, along with increasing physical demands on the pregnant body as delivery approaches. Each trimester carries different standard prenatal monitoring and testing recommendations, which is part of why tracking accurate week-by-week progress matters for coordinating care.
A due date represents an estimated midpoint rather than a guaranteed delivery day, and healthcare providers generally consider anywhere from 37 to 42 weeks to be a normal, healthy range for delivery timing. Factors like genetics, whether it's a first pregnancy, and individual variation in labor onset all play into exactly when delivery happens naturally. This is why due dates function best as a planning and monitoring anchor for scheduling checkups and tracking growth, rather than as a specific date to expect the birth to happen.
Can an ultrasound change my calculated due date? Yes — particularly a first-trimester ultrasound, which is often considered more accurate than a calculation based purely on your last period, especially if your cycles are irregular.
Why does week 4 of pregnancy feel confusing timing-wise? Because pregnancy dating starts from the last period rather than conception, the first two weeks technically occur before conception has actually happened — an artifact of the standard dating convention rather than an error.
Does this calculator work if I don't remember my exact last period date? It requires at least an approximate date to generate an estimate — if you're unsure, your healthcare provider can help establish more precise dating through an early ultrasound instead.
Is the due date the same as delivery date for a scheduled C-section? Not necessarily — scheduled C-sections are often planned at or near the due date, but the exact date depends on medical factors specific to each pregnancy and is set by your care provider.
What's the difference between gestational age and fetal age? Gestational age (the standard used by doctors and this calculator) is counted from the last menstrual period, while fetal age counts from actual conception, roughly two weeks less than gestational age at any given point.
Can this calculator estimate twins or multiples pregnancy dating? The dating math itself is identical, though multiples pregnancies are monitored somewhat differently and often deliver earlier than a typical single pregnancy on average.
How accurate is a due date calculated this way compared to an ultrasound? For someone with regular, well-tracked cycles, LMP-based dating is often reasonably close to ultrasound dating, though ultrasounds — particularly early ones — are generally considered more precise when cycle tracking is uncertain.
Should I tell my doctor which method I used to estimate my due date? It's helpful context, though your provider will typically confirm or adjust the date using their own clinical assessment and any ultrasound measurements taken during your first visit.
Last reviewed by Mehmed on July 11, 2026.