Pregnancy Due Date Calculator
Calculate the estimated due date, current gestational age, trimester and pregnancy progress using LMP, conception date, or IVF embryo-transfer dating.
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What is an Estimated Due Date?
The estimated due date (EDD) is the calculated date at approximately 40 weeks of gestation. It provides an estimated target date for delivery rather than a guaranteed date of birth.
Pregnancy dating is conventionally based on the first day of the last menstrual period when that date is reliable.
Naegele's Rule
A traditional method for estimating the due date is Naegele's rule.
EDD = LMP + 280 days
This corresponds to approximately 40 weeks from the first day of the last menstrual period.
The calculation assumes a typical menstrual cycle and ovulation pattern. Individual variation in cycle length and ovulation timing can affect the accuracy of LMP-based dating.
Due Date from Conception Date
When the date of conception is known or reliably estimated, the estimated due date can be calculated using approximately 266 days from conception.
EDD = Conception Date + 266 days
Conception-based dating should not be confused with gestational age, which conventionally begins approximately two weeks earlier than conception.
Pregnancy Dating After IVF
Assisted reproductive technology provides a more precise reference point because the embryo transfer date and embryo developmental age are known.
For IVF dating, the estimated gestational age is determined by accounting for both the embryo's developmental age at transfer and the conventional two-week pre-ovulation period used in obstetric dating.
| IVF Event | Approximate EDD Calculation |
|---|---|
| Day-3 embryo transfer | Transfer date + 263 days |
| Day-5 embryo transfer | Transfer date + 261 days |
IVF pregnancy dating should be documented according to the clinical dating convention used by the treating fertility or obstetric team.
Pregnancy Trimesters
| Trimester | Gestational Age | General Stage |
|---|---|---|
| First Trimester | 0 weeks to 13 weeks 6 days | Early pregnancy |
| Second Trimester | 14 weeks to 27 weeks 6 days | Mid-pregnancy |
| Third Trimester | 28 weeks onward | Late pregnancy |
Why Accurate Pregnancy Dating Matters
Establishing an appropriate gestational age is important throughout pregnancy because many clinical assessments are interpreted according to gestational age.
- Timing prenatal screening
- Interpreting ultrasound measurements
- Assessing fetal growth
- Interpreting maternal biochemical screening
- Planning antenatal investigations
- Monitoring pregnancy progression
- Supporting appropriate obstetric management
LMP Dating vs Ultrasound Dating
LMP-based dating works best when the menstrual history is reliable and cycles are reasonably regular.
When the LMP is uncertain, menstrual cycles are irregular, or there is a clinically significant discrepancy between menstrual dating and ultrasound measurements, ultrasound assessment may provide a more appropriate basis for pregnancy dating.
Early pregnancy ultrasound measurements are particularly useful for establishing gestational age when menstrual dating is uncertain.
Factors That May Affect Due Date Accuracy
| Factor | Effect on Dating |
|---|---|
| Uncertain LMP | May result in inaccurate LMP-based dating. |
| Irregular menstrual cycles | Ovulation may occur earlier or later than expected. |
| Variable ovulation timing | Conception may not occur around the conventional cycle day assumed by LMP dating. |
| Recent hormonal contraception | Menstrual timing may not reliably indicate ovulation. |
| Recent pregnancy | Menstrual patterns may be altered. |
Pregnancy Due Date Calculator - Frequently Asked Questions
How is the due date calculated?
When LMP is known, the estimated due date is conventionally calculated by adding 280 days, or 40 weeks, to the first day of the LMP.
What is Naegele's rule?
Naegele's rule is a traditional method of estimating the due date by adding approximately 280 days to the first day of the last menstrual period.
Can I calculate my due date from conception?
Yes. When conception is reliably known or estimated, approximately 266 days can be added to the conception date to estimate the due date.
How is an IVF due date calculated?
IVF dating uses the embryo transfer date together with the embryo's developmental age at transfer. Day-3 and Day-5 embryo transfers therefore use different calculations.
How many weeks is pregnancy?
The conventional pregnancy duration used for EDD calculation is approximately 40 weeks, or 280 days, from the first day of the LMP.
Is the due date the exact delivery date?
No. The estimated due date is a calculated reference point and does not guarantee the actual date of delivery.
What if I do not know my LMP?
If the LMP is uncertain or unreliable, clinical assessment and ultrasound-based dating may be needed.
Can the due date change after ultrasound?
Pregnancy dating may be revised when an appropriate ultrasound assessment indicates that the initial dating is unreliable or differs according to established clinical criteria.
Medical Disclaimer: This calculator is intended for educational and clinical reference purposes. The estimated due date is an approximation and depends on the accuracy of the dating information provided. LMP-based dating assumes that the menstrual date is reliable and that cycle characteristics are reasonably typical. When LMP is uncertain, menstrual cycles are irregular, IVF dating is involved, or clinical circumstances warrant, pregnancy dating should be assessed using appropriate clinical and ultrasound-based methods. This calculator should not replace professional obstetric assessment or individualized medical advice.
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