Due date (EDD): definition and meaning
A due date, or estimated due date (EDD), is the predicted date a baby will be born. It's usually set at 40 weeks (280 days) after the first day of the last menstrual period, which is about 38 weeks after conception. An early ultrasound can adjust it. Only about 4 to 5% of babies arrive on their exact due date.
How it's calculated
| Method | How it works |
|---|---|
| Last period (Naegele's rule) | First day of last period + 280 days, adjusted for cycle length |
| Ovulation or conception date | Ovulation + 266 days |
| IVF transfer | Transfer date + 266 days minus embryo age |
| Early ultrasound | Measuring the embryo, most accurate in the first trimester |
Try the due date calculator, or the IVF due date calculator after treatment.
Why it matters
Your EDD sets the pregnancy timeline: scan dates, screening tests, and when the trimesters begin. Doctors define full term as 39 weeks to 40 weeks 6 days, and most births happen within a couple of weeks either side of the date. Thinking of it as a due window takes some pressure off.
Because dating starts from your last period, it assumes ovulation around day 14. If your cycles are longer or shorter, the ultrasound may move your date, and that's normal. Knowing your cycle length and the date of your last period helps at the first appointment.
Book your first pregnancy appointment early. Get urgent care for heavy bleeding, severe belly pain, or one-sided pain in early pregnancy, which can be a sign of ectopic pregnancy.
Frequently asked questions
How accurate is a due date?
It's an estimate. Most babies arrive within two weeks of it, and a first-trimester ultrasound gives the most accurate date.
Can my due date change?
Yes, usually once after the early ultrasound if it differs from the period-based date by more than about a week.
Sources
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
Due date calculator: when is my baby due?Free pregnancy due date calculator: enter the first day of your last period and cycle length to see your estimated due date and how many weeks along you are.Due date calculator based on ovulation: when is my baby due?Due date calculator based on ovulation: enter your ovulation or conception date to get your estimated due date (ovulation + 266 days) and weeks pregnant.What is a trimester?A trimester is one of the three stages of pregnancy. Which weeks each trimester covers, what changes in each, and how weeks are counted from your last period.
Conception: definition and meaningConception is when sperm fertilizes an egg, the start of pregnancy. How it happens step by step, when it can happen in your cycle, and how long it takes.
Pregnancy calculator: how far along am I?Pregnancy calculator: enter your last period or due date to see how many weeks and days pregnant you are, your trimester and the days to go. Free and private.