Last-period dating
Count from the first day of your last menstrual period, then adjust for a cycle shorter or longer than 28 days.
Pregnancy dating estimate
Choose a dating method
Use the date that is most reliable for you. Your midwife or doctor can confirm pregnancy dating.
Use day one of your most recent period, not the day it ended.
Leave this at 28 days if you are not sure of your usual cycle length.
Planning estimate only. Pregnancy dating should be confirmed with a qualified clinician; this calculator is not medical advice.
Estimated due date
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Choose a method and enter a date.
Calculation method
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Estimated gestation
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At a glance
Pregnancy dating quick reference
These standard timing guides explain how the calculator creates an estimated due date.
Conception dating
Count from an estimated conception date, which is usually about two weeks after the last period in a 28-day cycle.
Gestational age
Pregnancy weeks are conventionally counted from the last period, rather than from conception.
IVF transfer
Use the transfer date and embryo age so the calculation can account for development before transfer.
How to use the due date calculator
Choose the dating method that matches the information you have.
1
Choose a method
Use Last period, Conception or IVF transfer.
2
Enter the date
Provide the relevant date and, where applicable, cycle length or embryo age.
3
Read the estimate
Use the single result panel and confirm the date with a qualified clinician.
Frequently Asked Questions
How is a due date estimated from the last period?
The calculator adds 280 days to the first day of the last menstrual period and adjusts for cycle lengths other than 28 days.
How is an IVF due date estimated?
The calculator adds 266 days from conception, subtracting the embryo age at transfer from the transfer-date interval.
Is the estimated date exact?
No. It is an estimate and should be confirmed by a qualified clinician, especially when dates or cycles are uncertain.
Why is pregnancy dated from the last menstrual period?
The first day of the last menstrual period is often easier to identify than conception. In a typical 28-day cycle, conception is estimated to occur about two weeks later, which is why a standard pregnancy is described as 40 weeks from the last period or about 38 weeks from conception.
What if my menstrual cycles are not 28 days long?
Enter your usual cycle length when using the last-period method. The calculator adjusts the estimate by the difference from 28 days. Irregular cycles can make any last-period estimate less certain, so discuss dating with your midwife or doctor.
Which date should I use if I know more than one?
Use the most reliable information available. A known IVF transfer date is usually more specific than a period date; otherwise use a known conception date or the first day of the last menstrual period. Your clinician can decide which information should be used for your care.
Can an ultrasound change my estimated due date?
Yes. An early dating ultrasound may provide a more reliable estimate when menstrual dates are uncertain, cycles are irregular, or the scan measurement differs meaningfully from the period-based calculation. Your maternity care team will explain the date used for your pregnancy care.
Does the due date predict the day my baby will be born?
No. It is a useful 40-week milestone, not a guaranteed birth date. Birth can happen before or after the estimate, and your care team will advise you about what is expected for your individual pregnancy.
How do I calculate a due date after IVF transfer?
Enter the embryo transfer date and select the embryo age at transfer. The calculator starts with 266 days from conception and subtracts the embryo age, so a 5-day transfer is estimated as transfer date plus 261 days.
When should I contact a healthcare professional?
Contact your midwife, GP, fertility clinic or maternity unit for personalised dating, pregnancy care, or any symptom that concerns you. This calculator is for general planning only and cannot assess your health or pregnancy.