Pregnancy Due Date Calculator
Estimate your pregnancy due date from your last menstrual period, conception or ovulation date, or IVF embryo transfer. See your estimated gestational age, trimester and key dates in one place.
If you're unsure, 28 days is the standard assumption.
Choose the dating method you know best and enter the relevant date to estimate your due date and pregnancy timeline.
How pregnancy due dates are calculated
A due date is an estimate of when a pregnancy reaches 40 weeks of gestational age. The calculation depends on which date you know best: the first day of your last menstrual period, an estimated conception or ovulation date, or the date and developmental age of an IVF embryo transfer.
Last menstrual period
The standard starting point is 280 days, or 40 weeks, from the first day of the last menstrual period. CalcNest also adjusts the estimate when a cycle length other than 28 days is entered.
Conception or ovulation
When conception timing is reasonably well known, the calculator adds 266 days. Gestational age is still expressed using the conventional pregnancy scale, which begins about two weeks before conception.
IVF embryo transfer
IVF provides a known transfer date and embryo developmental age. CalcNest subtracts the embryo's age to estimate fertilisation timing before applying standard pregnancy dating.
The maths behind the calculator
Pregnancy is conventionally dated from the first day of the last menstrual period rather than from conception itself. This means gestational age is already approximately two weeks when conception occurs in a typical 28-day cycle.
For menstrual-cycle dating, CalcNest starts with 280 days and shifts the estimate by the difference between the entered cycle length and 28 days. This accounts for the fact that ovulation does not occur on the same cycle day for everyone.
IVF dating works differently because the embryo's developmental age at transfer is known. A day-5 embryo, for example, represents approximately five days of development before the transfer date.
Menstrual dates can be uncertain and ovulation timing varies. Pregnancy dating may be revised after ultrasound assessment. If your maternity team has established an estimated due date for your pregnancy, follow that clinical date rather than replacing it with an online calculation.
Example due date calculation
Suppose the first day of the last menstrual period was 1 January 2026 and the average cycle length was 28 days. No cycle adjustment is required.
1 January 2026 + 280 days
Your due date is a reference point, not a prediction
The estimated due date represents the point at which pregnancy reaches 40 weeks. It does not mean labour or birth is expected to happen on that exact day.
| Result | What it means | Important context |
|---|---|---|
| Estimated due date | The estimated date at which gestational age reaches 40 weeks. | Birth commonly happens before or after this date. |
| Gestational age | How far the pregnancy has progressed using conventional obstetric dating. | It is not the same as time since conception. |
| Trimester | A broad way of dividing pregnancy into stages. | Clinical definitions and appointment timing can vary slightly. |
| Milestone dates | Calendar dates corresponding to selected gestational weeks. | They are planning references, not medical appointment recommendations. |
Early pregnancy ultrasound measurements can provide a more accurate estimate of gestational age than menstrual dates alone, particularly when cycle timing or the last menstrual period is uncertain.
A due date is an estimate and pregnancy can extend beyond 40 weeks. Follow the monitoring and advice provided by your maternity team rather than using this calculator to make care decisions.
Important limitations
CalcNest provides a calendar estimate using the information entered. It cannot confirm pregnancy, determine fetal development, assess whether a pregnancy is progressing normally, or replace antenatal care, ultrasound dating or advice from a qualified healthcare professional.
Pregnancy due date calculator FAQs
It provides an estimate based on the dates entered. Menstrual timing and ovulation can vary, so the result should not be treated as an exact prediction of birth. Ultrasound dating may lead your healthcare professional to establish a different estimated due date.
Gestational age is conventionally measured from the first day of the last menstrual period. In a typical 28-day cycle this is around two weeks before conception, which is why a full-term pregnancy is described as approximately 40 weeks even though conception occurs later.
It can. The standard LMP calculation assumes a 28-day cycle. CalcNest shifts the estimate by the difference between your entered cycle length and 28 days. This remains an estimate because the actual timing of ovulation can vary between cycles.
IVF dating can use the known embryo transfer date together with the embryo's developmental age. CalcNest estimates fertilisation by subtracting the embryo age from the transfer date and then applies standard obstetric dating.
Yes. A dating scan can provide a more accurate estimate of how far along a pregnancy is, particularly when menstrual dates are uncertain. Follow the estimated due date established by your maternity team.
Not necessarily. The due date represents the estimated point at which pregnancy reaches 40 weeks. Pregnancy commonly lasts between about 37 and 42 weeks, so birth may occur before or after the estimated date.
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