How your due date is actually worked out
A due date is not a delivery appointment. It is a single point estimate produced by a formula that assumes a textbook 28-day cycle and ovulation on day 14 — two assumptions that are true for only a minority of people. Understanding where the number comes from makes the rest of pregnancy much easier to interpret, because nearly every measurement, screening window and clinical decision over the next nine months is anchored to it.
The standard method is Naegele's rule, published by German obstetrician Franz Naegele in the early 1800s and still the worldwide default. It takes the first day of your last menstrual period (LMP), adds one year, subtracts three months, and adds seven days. Arithmetically that is identical to adding 280 days — exactly 40 weeks. This calculator does the day arithmetic directly, which avoids the rounding errors the "subtract three months" shortcut produces when the calendar crosses months of unequal length.
Why 280 days when pregnancy is 266 days?
This is the most common source of confusion and it is worth being precise about. A fertilised egg takes roughly 266 days to develop into a full-term baby. But you do not conceive on the first day of your period — you conceive around two weeks later, at ovulation. Obstetric dating starts from the LMP rather than conception because the LMP is a date you can actually observe and remember, whereas conception is invisible.
The consequence is that gestational age carries a two-week head start. On the day you get a positive test, about two weeks after conception, you are already counted as "four weeks pregnant" even though the embryo is only about two weeks old. Nobody is pregnant during weeks one and two of a 40-week pregnancy. This is a bookkeeping convention, not an error.
The five dating methods, and when each is the right one
Last menstrual period. The default. Reliable if your cycles are regular, you remember the date with confidence, and you were not on hormonal contraception in the three months before conceiving. Recall error is the weak point — studies of self-reported LMP consistently find a substantial share of people are off by several days.
Known conception date. If you tracked ovulation with LH strips, basal body temperature or a fertility monitor, or you know the single date of intercourse, dating from conception beats dating from LMP. The calculation is simply conception date plus 266 days.
IVF transfer. IVF produces the most precise dating available, because the embryo's age at transfer is known to the day. For a day-5 blastocyst transfer the due date is the transfer date plus 261 days; for a day-3 cleavage-stage embryo it is transfer date plus 263 days. Frozen embryo transfers follow the same rule based on the embryo's developmental day at freezing, not the calendar time spent in storage.
Ultrasound. A first-trimester scan measures crown-rump length, the distance from the top of the head to the bottom of the torso. Between roughly 8 and 13 weeks, embryos grow at a strikingly uniform rate, so this measurement dates a pregnancy to within about five days. Enter the weeks and days your sonographer gave you along with the scan date and this tool projects the due date forward.
When ultrasound overrides your period date
Professional guidance from ACOG and the American Institute of Ultrasound in Medicine sets specific thresholds for when a scan should replace an LMP-based date. The discrepancy required to justify a change grows as pregnancy progresses, because ultrasound dating loses precision later on — babies of identical gestational age vary far more in size at 30 weeks than at 10 weeks.
| Gestational age at scan | Redate if LMP and scan differ by more than |
|---|---|
| Up to 8w6d | 5 days |
| 9w0d – 15w6d | 7 days |
| 16w0d – 21w6d | 10 days |
| 22w0d – 27w6d | 14 days |
| 28w0d and beyond | 21 days |
Once a due date is established it is normally kept for the remainder of the pregnancy. Later scans measure growth against that fixed date rather than resetting it — otherwise a baby measuring small would simply be redated younger, and genuine growth restriction would be hidden.
How likely are you to give birth on your due date?
About 4 to 5 percent of babies arrive on the estimated date itself. That figure surprises people, but it follows directly from the arithmetic: spontaneous labour spreads across a window several weeks wide, so no single day can capture much of the distribution. Roughly half of first-time mothers deliver after 40 weeks and 5 days; the median is earlier in subsequent pregnancies. Around 90 percent of births happen between 37 and 42 weeks.
The practical implication is that a due date works better as the centre of a range than as a deadline. A more useful mental model is a "due month" — the four weeks from 38 to 42 — with the calculated date sitting near the middle of it.
Term definitions you will hear at appointments
In 2013 ACOG replaced the single label "term" with four categories, because outcomes at 37 weeks are measurably different from outcomes at 39 weeks. These labels come up constantly in conversations about scheduling induction or a planned caesarean.
| Label | Gestational age | What it means |
|---|---|---|
| Preterm | Before 37w0d | Higher risk of respiratory and feeding difficulty |
| Early term | 37w0d – 38w6d | Elective delivery generally avoided without a reason |
| Full term | 39w0d – 40w6d | Lowest rate of newborn complications |
| Late term | 41w0d – 41w6d | Increased monitoring typically begins |
| Post term | 42w0d and beyond | Induction usually recommended before this point |
Trimesters and what your due date unlocks
Trimester boundaries are conventions rather than biological switches, and different sources draw them a week apart. The most widely used split is: first trimester through 13w6d, second trimester 14w0d to 27w6d, third trimester 28w0d until birth. This calculator uses that split.
Your due date is the anchor for a long list of time-sensitive appointments. Being off by a week can push you outside the valid window for a screening test:
- 10–13 weeks — NIPT cell-free DNA screening; nuchal translucency scan
- 15–20 weeks — quad screen, if NIPT was not done
- 18–22 weeks — anatomy scan, the detailed structural survey
- 24–28 weeks — glucose tolerance test for gestational diabetes
- 28 weeks — anti-D immunoglobulin if you are Rh negative
- 27–36 weeks — Tdap vaccination, timed so antibodies transfer to the baby
- 36–37 weeks — group B strep swab
Working backwards: finding your conception date
Because the relationship between LMP, conception and due date is fixed arithmetic, any one of the three gives you the other two. If you only know your due date, conception was approximately 266 days before it, and your LMP was 280 days before it. The calculator displays the estimated conception date alongside your result for exactly this reason — it is frequently the number people are actually looking for.
Twins, multiples and other adjustments
Twin pregnancies are dated the same way, but they rarely reach 40 weeks. Average delivery for twins falls around 35 to 36 weeks, and many practices plan delivery at 37 to 38 weeks for uncomplicated dichorionic twins. Triplets average earlier still, near 32 weeks. The calculated 40-week date stays as the reference point clinicians use to measure growth, but nobody expects the pregnancy to reach it.
A previous pregnancy's length is a modest predictor of the next one. If you delivered spontaneously at 41 weeks the first time, you are somewhat more likely to go past 40 again. That is a tendency, not a rule, and it is not built into any standard formula.
A worked example
Suppose your last period started on 3 March and your cycles average 31 days with a normal 14-day luteal phase. Ovulation would fall around day 17, so conception is estimated at 20 March. Adding 266 days to that gives a due date of 11 December. A plain uncorrected Naegele calculation — 3 March plus 280 days — would have said 8 December, three days earlier. If a dating scan at 9 weeks then measured the pregnancy at 9w2d, the scan-derived date would be checked against 11 December; a gap of three days is well inside the seven-day threshold for that window, so the original date would stand.
Frequently asked questions
My cycles are irregular — is this calculator still useful?
Can my due date change later in pregnancy?
Why does my app show a different date to my midwife?
Does a positive pregnancy test tell me anything about dating?
Is 40 weeks measured from conception or from my period?
What if I was on the pill before conceiving?
Is my data stored anywhere?
Other tools on DueDatez
Checked against ACOG Committee Opinion 700 (term definitions) and ACOG/AIUM/SMFM guidance on estimating gestational age. This tool provides estimates for general information and does not replace advice from your midwife, obstetrician or physician.