Due date calculator

What a due date actually means

A due date, properly called the estimated date of delivery (EDD), is the single calendar day that marks 40 completed weeks of pregnancy. It is a useful anchor for scheduling scans, screening tests, and leave, but it is a centre point rather than a deadline. Most babies arrive in the weeks on either side of it. The American College of Obstetricians and Gynecologists describes a normal term pregnancy as anything from 37 weeks 0 days through 41 weeks 6 days, a span of five full weeks. Reading the due date as one day on a calendar misses how wide that healthy window really is, which is why this tool shows the whole term window rather than a lone date.

Naegele's rule and the 280-day count

The oldest and still most common method starts from the first day of your last menstrual period. Naegele's rule adds one year, subtracts three months, and adds seven days, which works out to 280 days. The reason the count begins before conception is that gestational age is measured from the last period, not from fertilisation. Ovulation and conception happen roughly two weeks into that count in an average 28-day cycle, so by the time a missed period prompts a test you are already considered about four weeks pregnant. If your cycles run longer or shorter than 28 days, the implied ovulation day moves, and adjusting the cycle length in the calculator above shifts your due date accordingly.

Why conception and IVF dating can differ

When you know more than just the last period, the estimate gets sharper. Dating from a known conception or ovulation date adds 266 days, because that method skips the assumed two-week gap between the last period and ovulation. IVF dating is the most precise of all, since the embryo's exact age is recorded in the clinic. A day-3 cleavage-stage transfer adds 263 days, a day-5 blastocyst adds 261 days, and a day-6 frozen blastocyst adds 260 days to the transfer date. These offsets are not arbitrary: each one counts back from the 280-day full-term figure by the embryo's age at transfer, so all the methods land on the same biological finish line from different starting points.

Reverse-dating from a due date you already have

If a clinician has already given you a due date, you can run this calculator in reverse. Choosing the known-due-date option recovers the implied last-period date, the likely conception window, and the calendar boundaries of each trimester. That is handy for filling in a pregnancy journal, planning the timing of the anatomy scan and glucose test, or simply understanding which week you are in on any given date. Because every method resolves to a single internal last-period date first, the trimester ranges and the term window stay perfectly consistent no matter which input you start from.

How the trimesters map onto your calendar

Pregnancy is divided into three trimesters by completed weeks counted from the last period. The first trimester runs from week 0 through the end of week 13, the second from week 14 through the end of week 27, and the third from week 28 to the due date. The calculator turns those week boundaries into concrete calendar dates for your pregnancy, so instead of doing the arithmetic in your head you can see, for example, exactly when your second trimester begins. Knowing those dates helps with the practical rhythm of care: first-trimester combined screening typically falls between 11 and 14 weeks, the detailed anatomy scan around 20 weeks, and the glucose tolerance test usually between 24 and 28 weeks.

Reading your term window

The term window section breaks the final stretch into the four labels obstetricians use. Early term covers 37 weeks 0 days to 38 weeks 6 days, full term covers 39 weeks 0 days to 40 weeks 6 days, late term covers 41 weeks 0 days to 41 weeks 6 days, and post term begins at 42 weeks. These distinctions matter because outcomes differ slightly across the window, and decisions about induction are often framed around them. The calculator prints the calendar dates for each band so you can see when your full-term window opens rather than fixating on the single due date.

Accuracy, ultrasound, and when to trust the dates

Treat every figure here as an estimate. The 280-day convention is built on average cycle length and average gestation, and real pregnancies vary widely. An early dating ultrasound, particularly one taken in the first trimester using crown-rump length, is more accurate than any calendar method and is the standard your care team will use, especially when the last-period date is uncertain or differs from the scan by more than about a week. Use this calculator to plan, to satisfy curiosity, and to understand the structure of your pregnancy, and let your OB-GYN or midwife confirm the clinical dating.