Health & Fitness
Pregnancy Due Date Calculator
Estimate a due date from your last period, conception, ovulation, or an IVF transfer.
Result
Enter your numbers and press Calculate to see the result.
Calculating…
How a due date is calculated
The standard method is Naegele's rule: add 280 days — 40 weeks — to the first day of your last menstrual period. It has been used since the early nineteenth century and remains the basis of every dating scan and maternity note.
due date = first day of last period + 280 days
The rule contains a deliberate quirk. Pregnancy is dated from the last period, not from conception, so the first two weeks of a "40-week pregnancy" happen before there is a pregnancy at all. Actual gestation from conception is about 266 days, or 38 weeks.
Dating from conception, ovulation, or IVF
Every method is converted to an equivalent LMP date first, then the same 280 days are added. That is why the results line up with what a midwife or clinic will tell you.
| You know | Adjustment | Days to due date |
|---|---|---|
| Last menstrual period | None | +280 from LMP |
| Conception or ovulation date | Count back 14 days | +266 from conception |
| IVF day-5 blastocyst transfer | Count back 19 days | +261 from transfer |
| IVF day-3 transfer | Count back 17 days | +263 from transfer |
IVF dates are the most accurate of all, because the moment of fertilisation is known rather than estimated. Frozen embryo transfers use the same rule, based on the embryo's age at the point it was frozen.
Cycle length changes the answer
Naegele's rule assumes a 28-day cycle with ovulation on day 14. Cycles longer than that ovulate later, which pushes the due date back:
| Cycle length | Adjustment to due date |
|---|---|
| 24 days | 4 days earlier |
| 28 days | No change |
| 32 days | 4 days later |
| 35 days | 7 days later |
On a 35-day cycle the uncorrected rule is a full week out — enough to affect decisions about induction near the end of the pregnancy, which is why the cycle length input matters.
How many weeks pregnant am I?
Gestational age is counted in completed weeks plus days since the LMP, written as "12w 3d". Trimesters divide as:
| Trimester | Weeks | Milestone |
|---|---|---|
| First | 0–13 | Dating scan usually 8–14 weeks |
| Second | 14–27 | Anatomy scan around 20 weeks |
| Third | 28–40 | Full term begins at 37 weeks |
Why the due date is an estimate
Only about 4% of babies arrive on their due date. Roughly 80% are born within the two weeks either side of it, and "full term" deliberately spans 37 to 42 weeks rather than naming a day.
An early ultrasound is more accurate than any date-based calculation, because early fetal growth is highly consistent between pregnancies. If a dating scan in the first trimester disagrees with the LMP estimate by more than about five to seven days, clinicians normally reset the due date to the scan.
This is a general estimate, not medical advice — your midwife, doctor, or clinic should confirm any date used for care decisions. To count the days until the estimated date, use the countdown calculator; for exact spans between any two dates, the date difference calculator.
Pregnancy Due Date Calculator — frequently asked questions
How is a due date calculated?
By Naegele's rule: add 280 days, or 40 weeks, to the first day of your last menstrual period. Pregnancy is dated from that day rather than from conception, which is why the first two weeks precede the pregnancy itself.
How do I calculate a due date from the conception date?
Add 266 days to the conception date, which is the same as counting back 14 days to an equivalent LMP and then adding 280. Ovulation dates work identically.
How is an IVF due date different?
IVF dating is the most accurate, because fertilisation happened at a known moment. A day-5 blastocyst transfer is 261 days to the due date; a day-3 transfer is 263.
Does cycle length affect the due date?
Yes. Naegele's rule assumes a 28-day cycle. A 35-day cycle ovulates roughly a week later, which pushes the due date a full week back — enough to matter near the end of the pregnancy.
How likely is the baby to arrive on the due date?
About 4% of babies are born on it. Roughly 80% arrive within two weeks either side, which is why full term spans 37 to 42 weeks rather than naming a single day.
Which is more accurate, a scan or the last period?
An early ultrasound. First-trimester fetal growth is very consistent, so if a dating scan disagrees with the LMP estimate by more than about five to seven days, clinicians usually reset the due date to the scan.