Formula and Method for the EDD Calculator (Estimated Date of Delivery)
The estimated due date (EDD) is the standard 40-week (280-day) prediction of when a pregnancy is expected to reach full term, counted from the first day of the last menstrual period (LMP). This is Naegele's Rule, the convention used by obstetricians and midwives worldwide: EDD = LMP + 280 days. If the conception or ovulation date is known instead, the calculation shifts to 266 days (38 weeks) from that date, since ovulation typically occurs about 14 days into a 28-day cycle. For IVF pregnancies the fertilization date is precisely known, so the due date is calculated from the embryo transfer date: 263 days for a day-3 cleavage-stage transfer, or 261 days for a day-5 blastocyst transfer.
How the calculation works
This calculator converts every method into an "effective LMP" date so gestational age and the due date use one consistent formula. For the LMP method, it also adjusts for cycle length: if your average cycle is longer or shorter than 28 days, the difference is added to or subtracted from the LMP before applying the 280-day rule, because ovulation shifts later in longer cycles and earlier in shorter ones. For conception-date and IVF-transfer inputs, the effective LMP is calculated by working backward 14 days from ovulation/fertilization (plus the embryo's age at transfer for IVF). The due date is then effective LMP + 280 days, and current gestational age is the number of days between the effective LMP and today, expressed in completed weeks and days.
Common mistakes
- Ignoring cycle length: Naegele's Rule assumes a textbook 28-day cycle with day-14 ovulation — a 35-day cycle pushes the real due date about a week later.
- Confusing gestational age with fetal (conception) age: gestational age is counted from the LMP, which is about two weeks before actual fertilization, so "8 weeks pregnant" means roughly 6 weeks since conception.
- Treating the EDD as a deadline: only about 5% of babies are born on their exact estimated due date; a full-term delivery window of 37–42 weeks is normal.
When to consult a healthcare provider
An ultrasound performed in the first trimester is generally considered the most accurate way to date a pregnancy and may adjust the EDD calculated here by several days. This tool provides an educational estimate based on standard obstetric conventions — it is not a medical diagnosis and does not replace prenatal care. Always confirm your dating and due date with your obstetrician, midwife, or another qualified healthcare provider, especially if your cycles are irregular or your dates are uncertain.