Due Date Calculator
Estimate a pregnancy due date from the last menstrual period or conception date, with the current week, trimester, and key milestones.
How to use this calculator
- 1Choose whether to calculate from your last period or a known conception date.
- 2Enter the date, and adjust the cycle length if yours isn't 28 days.
- 3Read the estimated due date, how many weeks along you are, and the milestone dates.
How it works
Naegele's rule
due date = first day of last period + 280 days (40 weeks) equivalently: LMP − 3 months + 7 days + 1 year adjust the start by (cycle length − 28) days from conception: conception + 266 days
The standard estimate counts 280 days — 40 weeks — from the first day of the last menstrual period, a method devised by the obstetrician Franz Naegele in the early 19th century. It assumes a regular 28-day cycle with ovulation on day 14, so pregnancy is dated from about two weeks before conception actually occurs. When the cycle is longer or shorter, or a conception date is known, the start is shifted accordingly, but the underlying 40-week count remains.
Worked example
For a last period beginning 1 May 2026 with a 28-day cycle, the due date is 280 days later — 5 February 2027. Conception is estimated around 15 May, two weeks after the period started, which is why pregnancy is counted as 40 weeks even though the baby develops for roughly 38.
Due Date Calculator: the complete guide
Why pregnancy is counted from before conception
One of the most confusing things about pregnancy dating is that the 40 weeks are counted from the first day of the last menstrual period — roughly two weeks before conception even happens. This is not a mistake; it is a practical convention. The date of the last period is something most people can identify, whereas the exact moment of conception usually cannot be pinned down.
The result is that 'weeks pregnant' runs about two weeks ahead of the baby's actual age. At the moment of a positive pregnancy test, often around week four or five, the embryo has only been developing for two or three weeks. The true gestational period from conception to birth averages about 266 days (38 weeks), while the clinical count of 280 days (40 weeks) starts earlier.
The due date is a midpoint, not a deadline
Naegele's rule produces a single date, but birth is a range. Only about one baby in twenty arrives on the exact due date; the rest spread out around it. A pregnancy is considered full term anywhere from 37 to 42 weeks, and a first baby in particular tends to arrive a little later than the estimate. The due date is best understood as the centre of a likely window rather than an appointment.
This matters emotionally as much as medically — treating the date as a deadline sets up disappointment when it passes, which it does for the majority of first pregnancies. Care providers monitor closely as the due date approaches and typically only intervene if the pregnancy continues past 41 to 42 weeks, when the risks of waiting begin to outweigh those of inducing labour.
When cycle length and ultrasounds refine the date
Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14, and many people don't fit that. A longer cycle means later ovulation and a later due date; a shorter one, the reverse. Adjusting the start by the difference between your cycle and 28 days, as this calculator does, corrects for that — a 32-day cycle pushes the due date about four days later.
The most accurate dating comes from an early ultrasound, usually in the first trimester, which measures the embryo directly and does not rely on knowing the cycle at all. If an ultrasound date differs significantly from the period-based estimate, providers generally use the ultrasound. Whatever a calculator says, your care provider's dating is the one that guides your care.
Frequently asked questions
How is a due date calculated?
By Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. Equivalently, subtract three months, add seven days, and add a year. The calculator adjusts this for your cycle length or a known conception date.
Why is pregnancy 40 weeks if the baby develops for 38?
Because dating starts from the last period, about two weeks before conception. That is the date most people can identify reliably, whereas conception usually can't be pinned down. So 'weeks pregnant' runs roughly two weeks ahead of the baby's actual developmental age.
How accurate is the due date?
It's an estimate — only about 4% of babies arrive on the exact date. A normal full-term birth happens anywhere from 37 to 42 weeks, and first babies often come a little later. An early ultrasound gives a more precise date than the period-based calculation.
What if my cycle isn't 28 days?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If yours differs, the due date shifts — a longer cycle means a later date. Enter your average cycle length above and the calculator adjusts the estimate accordingly.