Pregnancy Due Date Calculator

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Category Women's Health

Calculation Method

Select Date

Average Cycle Length (Days)

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About

Clinical estimation of the Estimated Date of Delivery (EDD) requires selecting the appropriate baseline event. While the standard medical model relies on the Last Menstrual Period (LMP), this method assumes a standard 28-day cycle and often yields inaccuracies for irregular cycles. Advanced calculation methods, such as those used for In Vitro Fertilization (IVF) or ultrasound dating, provide higher precision by anchoring the timeline to a known biological event rather than a historical calendar date.

This tool integrates multiple algorithms. For natural conception, it applies Naegele's Rule with cycle adjustments. For Assisted Reproductive Technology (ART), it utilizes the precise date of embryo transfer, adjusting for the embryo's age (Day 3 or Day 5 cleavage/blastocyst stage). Accuracy in these calculations allows for better monitoring of fetal development milestones and appropriate scheduling of prenatal screenings like the nuchal translucency scan or anomaly scan.

Formulas

The calculator employs specific logic based on the input method chosen:

LMP (Naegele"s Rule):
EDD = LMP + 280 days + (Cycle − 28)

IVF Transfer:
EDD = Date_transfer + 266 − Age_embryo

Ultrasound (Biometry):
EDD = Date_scan + (280 − GA_days)

Standard human gestation is defined as 280 days (40 weeks) from the LMP, or 266 days (38 weeks) from conception.

Reference Data

Milestone	Gestational Age	Description
First Trimester Ends	13 Weeks 6 Days	Organogenesis complete. Risk of miscarriage drops significantly.
Second Trimester Ends	27 Weeks 6 Days	Fetal viability increases; detailed anatomy scan window closes.
Full Term	37 Weeks 0 Days	Lungs considered mature; delivery safe without NICU typically.
Due Date (EDD)	40 Weeks 0 Days	Standard reference point for post-dates induction.
Post Term	> 42 Weeks	Increased monitoring required for placental insufficiency.

Frequently Asked Questions

LMP assumes ovulation occurs exactly on day 14 of a 28-day cycle. If ovulation occurred earlier or later, the LMP date will be incorrect. Ultrasound measures physical fetal size (Crown-Rump Length), which is a more objective indicator of gestational age in the first trimester. Medical protocols usually prioritize the ultrasound date if the discrepancy exceeds 7 days.

IVF dating is precise because the exact moment of fertilization (or transfer) is known. There is no biological ambiguity regarding ovulation timing. The due date is calculated by adding 266 days to the fertilization date, or adjusting the transfer date by subtracting the embryo age (3 or 5 days).

Yes. If calculating by LMP, a cycle longer than 28 days suggests ovulation occurred later than standard. The formula adds the difference (e.g., a 32-day cycle adds 4 days to the EDD). Conversely, shorter cycles subtract days.