Pregnancy Due Date Calculator
Estimate your due date and current gestational age from the first day of your last menstrual period.
This estimate uses Naegele's rule, which assumes a standard 28-day cycle — it's a general estimate, not a diagnosis. Only about 1 in 20 babies arrive on their exact due date. Actual timing varies, so always follow the dates and guidance your doctor or midwife gives you based on your own pregnancy.
How to use the Pregnancy Due Date Calculator
- Enter the first day of your last menstrual period.
- Read the estimated due date, 280 days (40 weeks) later.
- Check the estimated gestational age today, in weeks and days, and confirm dates with your doctor or midwife after your first scan.
About the Pregnancy Due Date Calculator
This calculator estimates a pregnancy due date using Naegele's rule, a standard method that adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). It's the same basic calculation used as a starting reference point in many prenatal care settings, before an ultrasound provides a more individualized estimate.
Enter the first day of your last period and the tool returns an estimated due date, along with the current estimated gestational age in weeks and days if that date has already passed. This is a general estimate based on a standard 28-day cycle assumption — actual due dates vary, and this tool does not replace guidance from a doctor or midwife, who can refine the estimate using ultrasound dating and your individual cycle history.
Worked example
Example: a last menstrual period starting January 1, 2026 gives an estimated due date of October 8, 2026 — 280 days later.
Naegele's rule by hand
Adding 280 days to the first day of the last period is the modern form of Naegele's rule. The classic shortcut gives almost the same date: take the first day of the last period, subtract three months, add seven days, and move to the following year. For a last period starting January 1, three months back is October 1, and adding seven days gives October 8 — the same date this calculator shows.
Milestones along the way
Pregnancy is usually counted in weeks and days from the last period, written like 12+3 (12 weeks and 3 days), which is how the gestational age is shown here. Common reference points:
- First trimester: from the start through week 13.
- Second trimester: weeks 14 to 27.
- Third trimester: week 28 until birth.
- Early term is 37 to 38 weeks, full term 39 to 40 weeks, late term 41 weeks, and post-term 42 weeks or more.
When the estimate may need adjusting
The standard calculation assumes a 28-day cycle. With regular longer or shorter cycles, providers sometimes shift the date by the difference: a consistent 32-day cycle moves the estimate about four days later. Irregular cycles, recent hormonal contraception, or uncertainty about the last period make the date less reliable, which is why an early ultrasound often becomes the reference. For pregnancies after IVF, the due date is calculated from the embryo transfer date instead. Always use the date your doctor or midwife gives you for planning and care.
Frequently asked questions
- How accurate is a due date calculated this way?
- It's a general estimate, not a precise prediction — only about 1 in 20 babies are born on their exact calculated due date, and full-term birth is considered normal anywhere from about 3 weeks before to 2 weeks after it. Naegele's rule also assumes a standard 28-day cycle, so it's less accurate for people with longer or shorter cycles. Early ultrasound dating is generally considered more precise than LMP-based calculation alone.
- What if my menstrual cycle isn't exactly 28 days?
- Naegele's rule assumes a 28-day cycle with ovulation around day 14, so if your cycles are consistently longer or shorter, the actual due date may shift earlier or later than this estimate suggests. A healthcare provider can adjust the estimate based on your typical cycle length or an early ultrasound.
- Does this replace a doctor's estimate?
- No. This tool provides a quick, general estimate using a standard formula, but it isn't a substitute for prenatal care. Always follow the due date and guidance given by your doctor or midwife, who can account for your individual health history and ultrasound measurements.
- What is gestational age, and how is it different from fetal age?
- Gestational age is measured from the first day of the last menstrual period, which is roughly two weeks before conception actually occurs — this is the standard convention used in prenatal care. Fetal age (or 'conceptional age') measures from the estimated date of conception instead, so it typically runs about two weeks behind gestational age for the same point in a pregnancy.