📋 In This Guide
How Due Dates Are Calculated
An estimated due date (EDD) is calculated using one of a few standard methods depending on what information is available — the first day of your last menstrual period, an estimated conception date, or in the case of IVF, the exact known embryo transfer date. This calculator supports all three.
LMP-Based
Most common method — adds 280 days to your last period's start date
Conception-Based
Adds 266 days from an estimated ovulation/conception date
IVF Transfer-Based
Most precise — based on the embryo's exact known age
Naegele's Rule Explained
Naegele's rule, developed in the 19th century and still the standard method today, adds 280 days (40 weeks) to the first day of your last menstrual period. It assumes a typical 28-day cycle with ovulation occurring around day 14 — which is why the "40 weeks of pregnancy" you often hear about actually includes roughly two weeks before conception even occurred.
Gestational Age vs Fetal Age
Gestational age — the number doctors and this calculator use — is counted from the first day of your last period, about two weeks before conception actually happens. Fetal age (or conceptional age) counts from the actual date of conception instead, running about two weeks behind gestational age. Medical convention almost always uses gestational age, which is why "40 weeks pregnant" doesn't mean the baby developed for a full 40 weeks from conception.
Need to calculate someone's exact age instead?
Calculate exact age in years, months and days for any date.
Calculating Due Dates From IVF Transfer
IVF pregnancies allow for unusually precise due date calculation, since the embryo's exact developmental age is known at the moment of transfer.
| Transfer Type | Formula |
|---|---|
| Day 5 (Blastocyst) Transfer | Transfer Date + 261 Days |
| Day 3 Transfer | Transfer Date + 263 Days |
How Accurate Is a Due Date, Really?
Despite the precision implied by a specific calendar date, only around 5% of babies are actually born on their calculated due date. Most arrive somewhere within a two-week window before or after — which is why medical professionals increasingly describe it as an "estimated" due date and plan around a range rather than a single fixed day.
5 Things to Know About Due Dates
Frequently Asked Questions
Naegele's rule adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle with ovulation around day 14.
It's an estimate — only about 5% of babies are born exactly on their calculated due date. Most arrive within a two-week window on either side.
Gestational age counts from the last menstrual period, about two weeks before conception. Fetal age counts from actual conception. Doctors conventionally use gestational age.
First trimester: weeks 1-12. Second trimester: weeks 13-27. Third trimester: weeks 28 through birth, typically around week 40.
For a Day 5 blastocyst transfer, add 261 days to the transfer date. For a Day 3 transfer, add 263 days — accounting for the embryo's known developmental age.
An early ultrasound measuring the fetus directly is often considered more accurate than LMP-based dating alone — doctors may adjust the due date if it significantly differs.
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